Different Cleaning Techniques for Archeological Ceramics: A Review

Abstract

Archeological ceramics represent values that necessitate preservation from various factors of deterioration. Cleaning processes are beneficial in the preservation of these ceramics. An abundance of cleaning technique and process information exists within the literature. This study examines the current state of both traditional and advanced cleaning techniques employed for archeological ceramics. The review discusses a wide range of commonly used cleaning techniques, including mechanical, dry and wet processes, as well as chemical approaches. Additionally, more recent laser, plasma, and biocleaning methods are discussed. The effectiveness of these techniques is examined, as well as potential damage or surface modifications to the ceramics. The selection of a cleaning method for ceramics depends on the specific characteristics of the ceramic (i.e., porosity, glaze, slip red-slipped, etc.), its state of conservation, and the nature and thickness of the fouling or encrustations. Careful selection and testing of chemical solutions are crucial to prevent damage. While chelating agents like EDTA effectively dissolve crusts and salts, uncontrolled application can weaken ceramic structures. Laponite, natural clay minerals, resins and organic gels (xanthan gum, agar, cellulose powder) are effective in removing contaminants from the surfaces of without causing damage. Environmentally friendly methods such as biocleaning, Pulsed Laser Cleaning, and plasma are effective but underutilized, requiring further investigation. This review emphasizes the growing potential of sustainable and non-invasive methods to complement or replace traditional approaches. Its main contribution lies in providing a critical synthesis that bridges conventional and innovative techniques, outlining research gaps for more effective and eco-responsible conservation of archeological ceramics.

1. Introduction

Archeological ceramics are often exposed to a wide range of exogenous materials originating from burial environments, seawater immersion, or post-depositional aging and degradation processes. These degradation factors, including physical, chemical, biological, and environmental ones (such as air pollution), lead to the accumulation of extraneous substances such as encrustations, soot, deposits, and dirt on their surfaces [1,2,3,4,5]. Contaminants may also derive from past restoration or museum practices, such as old adhesives, coatings, or chemically active inks used for inventory numbers [6]. Such contaminants can chemically and physically interact with the ceramic matrix, causing irreversible surface alterations and accelerating deterioration [7,8]. The presence of these contaminants not only obscures decorative and structural details but also threatens the material’s long-term stability. Understanding the sources and types of contaminants, as well as their harmful effects, is therefore fundamental to planning appropriate conservation strategies.

The main aim of conservation treatment is to ensure the chemical and physical stability of the object while maintaining its historical, esthetic, and scientific values. Cleaning, as a key phase of this process, plays a decisive role in mitigating deterioration and improving the legibility of artifacts [7,8]. However, cleaning is also one of the most delicate and controversial procedures in conservation, particularly when dealing with thick encrustations or hard deposits whose removal might cause the loss of original material. This dilemma, between eliminating harmful crusts and preserving the artifact’s integrity, represents a central challenge in archeological ceramic conservation [9].

To minimize risks, cleaning must be preceded by a thorough analytical assessment. Before any intervention, it is essential to determine the type of ceramic (e.g., glazed, decorated, or painted), its state of preservation, mineralogical composition, and the physicochemical nature of the dirt and deposits [9,10,11,12,13,14,15]. Without this preliminary analysis, there is a significant risk of removing original material during treatment. Thus, a scientific approach combining diagnostic analysis, material characterization, and controlled cleaning procedures is required to ensure both effectiveness and safety [7,16].

Over the years, various cleaning techniques have been developed and refined to address the specific challenges posed by these fragile materials. The choice of method depends on the chemical, physical, and textural properties of both the material to be removed and the object to be cleaned (

Table 1. The most common types of fouling, their composition, and techniques for removal.

Fouling Type	Composition	Formation	Removal	References
Insoluble salts	Mainly composed of calcium carbonate, but they can also contain low amounts of silicate, chlorides, sulfates, and phosphates	Formed during long-term burial	Mechanical and chemical cleaning methods, including the use of acids and chelating agents	[9,15,39,43]
Soluble salts	Mainly halite, gypsum, sodium sulfate, potassium sulfate, potassium chloride, sodium nitrate, potassium nitrate, sodium carbonate, potassium carbonate	Groundwater and seawater can carry these salts into the pores of the artifact during burial	Poultices and deionized water used in static immersion or ultrasonic baths	[44]
Mineralogical alteration product	New formed minerals, the transformation of existing minerals, and the dissolution of minerals, including carbonates and silicates	Formed during long-term burial or as a result of uncontrolled restoration interventions	Mechanical and chemical cleaning methods	[45]
Chemical alteration	Leached elements and oxidation or reduction products	Formed during long-term burial or as a result of uncontrolled restoration interventions	Chemical cleaning methods, including the use of solvents and enzymes	[45]
Organic	Lipids (fats, waxes, and resins), proteins, carbohydrates, and nucleic acids	Absorbed into the ceramic material during use, or deposited on the surface of the object over time		[46,47]

) [9,11,12,15,17,18,19]. Traditional cleaning methods include mechanical (e.g., brushing, rubbing, washing, steaming) and wet or chemical approaches (e.g., alkaline, acidic, or solvent-based treatments). Recent advancements have introduced innovative and less invasive alternatives, including laser [20,21,22,23,24,25], plasma [10,11,26,27,28,29,30], gels [4,7,13,15,31,32,33,34,35,36], ultrasonic [14,37], poultices [3,9,35,38,39] and refiring [7,40]. Alternative and less aggressive techniques like laser, biological (i.e., microbes and enzymes) and innovative biotechnological (i.e., pelargonic acid, and Bacillus-based cleaning products) cleaning are increasingly being adopted to reduce the risk of the loss of original material [24,25,41,42]. The effectiveness of any cleaning technique depends on several factors, including the state of degradation, the time and resources required, as well as the desired results in relation to the objective for which the cleaning was carried out. Therefore, cleaning must be regarded as a scientifically informed, context-dependent decision rather than a routine procedure.

2. Research Aim

The increasing amount of data published over the past four decades on the cleaning of archeological and historical ceramics has offered a comprehensive perspective on the preservation methodologies employed for this cultural heritage. Despite this, no review study focusing specifically on these cleaning techniques has been published to date. The aim of this review is to integrate and systematize published data on the cleaning of archeological ceramics in order to provide a critical analysis of the existing methods, compare their efficacy, and formulate recommendations for their appropriate and sustainable application in conservation practice. The specific objectives are:

• To classify traditional and advanced cleaning techniques according to their principles and materials.
• To evaluate and compare the effectiveness, advantages, and potential risks of each method.
• To identify research gaps and limitations in current cleaning approaches.

3. Degradation Processes Affecting Archeological Ceramic

Archeological ceramics are primarily fired objects or fragments made from clay, often containing varying proportions of lithic inclusions and other materials. Chemically, they predominantly consist of SiO₂ and Al₂O₃, accompanied by oxides such as CaO, Fe₂O₃, TiO₂, K₂O, and Na₂O. Found across diverse archeological contexts, including terrestrial and aquatic environments (both freshwater and saltwater), these artifacts encompass a wide range of forms, such as pottery, bricks, and tiles, reflecting their multifaceted utility in historical societies [48].

Archeological ceramics can suffer from deterioration and transformation processes that affect their color, weight, composition, and size due to aggressive burial environments. Ceramics, composed of mineral phases and sometimes an amorphous phase, have open porosity that partially communicates with the external environment. In the burial environment, fluids circulate in contact with both the external surface and the internal portions of the ceramics, depending on the ceramic’s microstructure.

Various physico-chemical and biological factors can deteriorate ceramics. Physically, plant root penetration, freeze/thaw cycles, abrasion, and crystallization/hydration cycles can cause cracking, spalling, or structural disintegration [8,49]. Chemically, groundwater and soluble salts are the main factors in the chemical deterioration of ceramic artifacts. Soluble salts, such as chlorides, sulfates, and nitrates, can migrate through the porous structure and, upon crystallization or hydration cycles, exert crystallization pressure that leads to microcracking, spalling, and surface flaking, thus directly threatening the structural integrity of the ceramics. Their presence explains the complexity and necessity of carefully controlled cleaning to avoid further damage. These salts can also react with metal components and organic acids from the soil, altering surface and chemical composition of the ceramics [49,50]. Biologically, microbial activity and plant roots can penetrate and degrade the ceramic structure.

These transformation processes also lead to the appearance of stains on the surface of ceramics, mainly resulting from interactions with soil and environmental factors. Ceramics in contact with deposits containing decaying vegetation can exhibit blue-black stains, chemically similar to old-fashioned iron gall ink, due to the reaction of tannic acids with iron compounds [51]. Additionally, iron stains can form from soil iron oxidation and salt efflorescence, where soluble salts leave white or gray deposits upon moisture exposure.

Post-depositional transformations, including secondary phase precipitation, mineral dissolution, conversion of original minerals, and chemical leaching and enrichment, can further damage ceramics [52]. Furthermore, all these deterioration mechanisms are intensified after excavation due to the presence of extrinsic factors such as temperature, relative humidity, air pollution, light, and inappropriate storage [2]. These combined degradation processes underscore the critical need for carefully planned cleaning strategies, as inappropriate or aggressive interventions can further increase the damage caused by soluble salts and other deteriorative agents.

4. Cleaning Methods

4.1. Mechanical Cleaning

Mechanical cleaning involves the physical removal of unwanted deposits from the surface of ceramic artifacts using various tools. Mechanical cleaning techniques are commonly adopted to remove more stubborn dirt, thick encrustations and hard deposits avoiding any surface damage that may produce material and/or information loss [9]. Conservators utilize small tools such as scalpel blades, bamboo picks, or wooden tools to carefully target specific areas for dirt removal [18]. Mechanical cleaning is more effective for ceramic with low porosity [14]. These methods require precision and a keen understanding of the ceramic material to avoid causing unintended damage during the cleaning process. However, mechanical cleaning, can generally be better controlled than chemical methods and there is no danger of dirt being drawn in solution into the ceramic body [7].

Prior to any cleaning intervention, the ceramic objects must be examined to identify the type of dirt, stains, or encrustations present. The fragility of the ceramics should also be evaluated through macroscopic observation and microscopic examination under a stereomicroscope or binocular microscope, combined with gentle tactile assessment to detect cracks, powdering, or weak cohesion. This evaluation helps determine the most suitable cleaning tools and techniques. These methods may include dry cleaning, which not involves the use of liquids, and wet cleaning, which use liquids (i.e., brushing, rubbing, the use of scalpels, bamboo picks, wooden tools, erasers, and sponges).

4.1.1. Dry Cleaning

Dry cleaning is the initial step in the cleaning process, involving the gentle removal of loose soil, not greasy dirt, dust not strongly adhered to the surfaces and debris from the ceramic surface using soft brushes, sponges, compressed air or vacuum cleaner. This technique minimizes the risk of surface scratching or damage and prepares the artifact for further cleaning procedures [7]. However, dry cleaning methods are usually excluded if the object is in an advanced state of deterioration.

4.1.2. Wet Cleaning

Wet cleaning involves the application of water or other liquid cleaning agents to dissolve dirt and stains that cannot be removed through dry cleaning. Commonly used agents include distilled water, mild soaps, or specially formulated detergents that not react with the ceramic material. Wet cleaning should be performed with caution, to prevent excessive moisture exposure or prolonged immersion, which can cause swelling, color loss, or structural damage to the ceramic. In particular, low-fired pottery contains unfired clay areas that can rehydrate and swell if subjected to prolonged wet condition resulting in loss of some elements during prolonged contact with water during conservation treatment [51,53,54]. Furthermore, high-fired earthenwares may contain, as body fillers, mineral particles, some of which may soluble in water. For these reasons wet cleaning must be locally used in a limited range [7].

4.1.3. Cotton Wool

Cotton wool is a cellulose material obtained from the Gossypium sp. seeds with a characteristic microstructure that makes it very resistant, malleable, soft, and very absorbent. Cotton wool is used to gently remove dirt and deposits from the surface of ceramic artifacts, without applying excessive force to prevent abrasion or damage. In excavated pottery pots, dusting was carried out using a brush of appropriate size and dry cotton wool swabs avoiding the solubility of adhesive used in joining pottery fragments. Cotton swabs are recommended for porous bodies with iron and carbon stains to prevent the spread of these stains caused by water movement [7].

4.1.4. Ultrasonic Cleaning

Ultrasonic cleaning uses high-frequency sound waves to create microscopic cavitation bubbles in a liquid cleaning solution, typically distilled water. When these bubbles collapse, they produce tiny, powerful jets of liquid that dislodge contaminants from the surface of the ceramics. Ultrasonic was used, prior to chemical cleaning, to clean underwater archeological ceramics from south Italy [1,14]. An ultrasonic piezoelectric device (micromotor ART 6000 CTS, CTS srl, Briosco, Italy) operating at rotational speeds between 9000 and 15,000 r.p.m. was gently applied to remove the encrustation and algae from underwater ceramics (Figure 1). Its seem to remove easily algae compared to other more encrusting organisms [14]. However, ultrasonic cleaning can pose significant risks to fragile ceramics due to the forces generated by cavitation. The violent collapse of cavitation bubbles can cause microcracks, erosion or exacerbate existing fractures in ceramic materials. Additionally, the liquid jets produced during cavitation may detach surface particles, especially on decorated or painted ceramics, leading to a loss of original material. To mitigate these risks, using low-intensity ultrasound or degassed water has been suggested, as these approaches reduce the aggressive effects of cavitation [55]. It is advisable to proceed cautiously, conducting preliminary tests before applying ultrasonic cleaning to archeological ceramics.

4.1.5. Dry Blasting

Dry blasting involves the use of abrasive particles propelled at high speed to clean the surface of ceramic artifacts. This method is effective for removing tough encrustations and deposits without the use of water or chemicals. Dry blasting with various abrasive particles, including sand, alumina and peach pit, was tested to remove the incrustation layers from glazed ceramics using a micro-blasting jet. This method proved to be harmful on glazed ceramic. Therefore they could only be used to reduce the thickness of the incrustation without reaching glazed surface [56]. Dry blasting, while effective, can cause significant degradation to fragile archeological pieces. Abrasive particles used during the process may remove, or substantially diminish fine surface details and decorations, and even portions of the original material. The high-speed impact of the particles can also weaken the pre-existing microfractures.

4.2. Chemical Cleaning

Chemical cleaning techniques may be employed in cases where mechanical techniques are less efficient or deposits require intensive cleaning. These techniques involve the use of acidic or alkaline solutions to dissolve and remove mineral salt deposits or corrosion products that obscure the surface of ceramics, making them hard to reconstruct [57]. Particularly, crystallization of soluble and insoluble salts is a common issue in the conservation of archeological ceramics. These mineral salts are commonly carbonate or sulfate salts including caliche, lime, marl, insoluble salt and calcareous crust. Soluble salts can be easily removed by immersion in water. However, the removal of insoluble salts, such as calcium carbonate, is more challenging. These salts are difficult to remove mechanically or dissolve in water due to their low solubility [57]. Traditional methods for their removal involve immersing the ceramic in a dilute acid solution, which changes the salts into a form that is water-soluble [43]. Carbonates are easily acid-soluble contrary to sulfates.

4.2.1. Acid Solutions

Types of Acids and Their Efficacy

Inorganic and organic acid solutions, such as hydrochloric acid (HCl), nitric acid (HNO₃), formic acid (HCOOH), oxalic acid ((COOH)₂), or acetic acid (CH₃COOH), are commonly and effectively used to dissolve calcium carbonate deposits and encrustations on ceramic surfaces [7,9,39,58,59]. The effectiveness of acid cleaning depends on several factors, including the acid concentration, the method of application (immersion or poultice), the duration of exposure, and the composition of the deposits. Both acetic and nitric acid at 1%, applied by immersion, have proven to be effective in removing calcareous deposits without altering the initial mineralogical composition of the ceramic specimens [43]. Dilute acid solutions, such as a solution of 1.5–10 M oxalic acid, are used to release insoluble Ca⁺ and Mg⁺ ions from calcareous deposit on the ceramic surface by immersing [58,59]. Chemical cleaning provides better results in ceramics with high porosity, by facilitating a deeper cleaning [14].

Risk Factors

High concentrations of acids and aggressive procedures can effectively remove acid-soluble deposits but also dissolve acid-soluble components of the ceramic, such as calcite, dolomite, and ankerite, potentially causing micro-physical damage and loss of surface components, slip, and surrounding clay [60]. Cleaning with 5–25% organic (CH₃COOH) and inorganic (HNO₃) acids for 2 min is the most aggressive procedure [39]. HNO₃ applied in an agar-agar gel medium caused material loss and morphological and structural alteration of the glazed ceramic [56]. The interaction of iron, which is present in clay and coatings, with hydrochloric acid can result in the formation of iron chloride, iron oxide, and yellow-colored substances [57,61]. In addition, the redeposition of insoluble salts within the pores can occur during the cleaning process, posing a potential risk of additional damage if not effectively removed [43,62].

To avoid these issues, ceramics should be soaked in tap water before being immersed in a 2.5% HCl solution until no bubbles appear on the surface. This process fills the pores and slows the diffusion of H⁺ and Cl⁻ ions into the ceramic body [18]. The deposition of salts during the cleaning process can be reduced by soaking the ceramic in water before immersing it in acid [43,62].

Careful selection of compatible cleaning solutions, thorough testing, and precise control of exposure time are essential to prevent adverse effects (Figure 2) [9,43,63]. Alternative treatments or less aggressive acids may sometimes be more suitable.

Specific Case Studies

Harrision (2008) [61] showed that treatment with 10% acetic acid induced the fewest changes in red-slipped sherds from the Assyrian Colonial Period, while 4% HCl and 4% HNO₃ resulted in greater losses of Ca, P, K, S, and As. A study on painted archeological ceramics from Los Villares de Andújar (Jaén, Spain) showed that treatment with 5% nitric acid, applied by immersion or using a cellulose pulp poultice, caused surface alterations such as cracks, increased roughness, and formation of thin white layers likely composed of calcium carbonate or calcium nitrate [63] (Figure 2).

4.2.2. Oxidizing Agents

Agents Used and Their Efficacy

Oxidizing agents, such as hydrogen peroxide (H₂O₂), are commonly used to degrade organic residues and/or reduce stains, which often consist of mixtures of organic and inorganic constituents, that are held within ceramic bodies [51,59]. The organic residues may include proteins, lipids, and other carbon-based materials. This oxidizing agent chemically alters organic residues, making them more polar upon oxidation, and therefore more water-soluble, as well as decolorizing them. This process breaks down these substances into smaller molecules, separating them from the ceramics.

A solution of 30–20% H₂O₂ applied by immersion for 24 h has been used for the removal of solid organic compounds, such as soot, without damaging the ceramic material [58,64]. Pouliot et al. [65] attested that hydrogen peroxide performs well when used at lower concentrations (~5% v/v) for general applications, or progressively at higher concentrations (10–15%) for more stubborn or specific stains.

Carbamide peroxide, a hydrogen peroxide stabilized by urea (CH₄N₂O), ensures a slow release of hydrogen peroxide, offering greater uniformity and efficiency in reducing organic stains [66]. It decomposes gradually into hydrogen peroxide, water, and oxygen, while urea breaks down into ammonia and carbon dioxide. A 20% solution of carbamide peroxide at pH 8, applied via agarose gel blocks, effectively reduced organic stains after two 40 min treatments. However, the treatment effect was not homogeneous among the sherds [67].

Risk Factors

Oxidizing agents can potentially oxidize metallic components, unstable enamels, metallic glazes, or gilding, which may result in color changes or surface damage [51,68]. H₂O₂ decomposes rapidly; therefore, controlled poultices are recommended to prevent tide-lines and redeposition of residues [65].

4.2.3. Chelating Agents

Agents Used and Their Efficacy

Chelating agents, such as ethylenediaminetetraacetic acid (EDTA), are widely used to remove calcareous encrustations, hard deposits, and soiled surfaces on archeological ceramics [69]. The efficiency of a chelating agent often depends on the pH of the surrounding solution. Chelation is most effective when at least three acid groups in the chelator are ionized, which occurs near their pKa values, and when the pH is close to the ceramic’s point of zero charge (pHzpc). Additionally, chelators exhibit selectivity, preferentially binding to certain metals based on their ionic radius and charge. For example, ethylenediaminetetraacetic acid (EDTA) forms stronger complexes at neutral to alkaline pH and has a higher affinity for calcium and iron than for sodium.

EDTA (CH₂N(CH₂COOH)₂) has been used since the 1980s as a safe and effective alternative to other chemical treatments, such as tribasic sodium phosphate [15,69]. is widely used to reduce oxidation stains and remove soiled deposits, particularly calcareous encrustations and hard deposits on ceramic shard surfaces, which developed through chemical dissolution and leaching processes [18,56]. A 10% EDTA solution, prepared in acetone, was used to clean ceramic surface. It was applied using a cotton poultice, left for 2 h, and then reapplied for an additional 4 and 8 h under a polyethylene sheet [14]. Alkaline solution of 5–15% of disodium EDTA (Na₂EDTA) at pH = 11.5 applied with cotton or cellulose pulp poultices, replaced every 10 min for a total of 30 min, effectively removed insoluble Ca²⁺, Mg²⁺, and Fe³⁺ salts from sediment covering underwater archeological ceramics [3,14,58,70]. After 24 h of application, this chelating treatment help to easily remove the hard crusts mechanically using fine brushes and wooden tools [14,70].

Tetrasodium EDTA (Na₄EDTA), at 1% w/v in deionized water, applied by immersion and thickened with xanthan gum powder, was effective in removing calcareous deposits from ceramic specimens fired from a temperature of 650 °C to 1100 °C, without decreasing the calcium carbonate content of non-carbonated specimens, although minor leaching of Fe and Ca was observed [15]. Soluble salts were effectively removed using EDTA by immersion, cellulose pulp poultice, and a xanthan gum (Vanzan^® NF-C, Vanderbilt Minerals, Inc., Norwalk, CT, USA) gel. Among these methods, immersion and cellulose pulp poultice were more effective than gel treatments [39]. The reduced effectiveness of the gel treatment may be attributed to its slower, controlled release of the cleaning agent [15,71,72]. Prior to immersion in the EDTA solutions, the ceramics were soaked in a deionized water bath for 24 h [39].

Risk Factors

While EDTA is highly effective at dissolving calcareous crusts and removing soluble salts, it can slowly infiltrate and weaken structures of ceramics by dissolving calcareous components and leaching metallic oxides. It may also sequester iron or manganese ions from painted decorations, potentially changing their color and artistic value [18]. Even when applied in an agar-agar gel medium, disodium EDTA caused material loss and morphological and structural alteration of the glazed ceramic [56].

Specific Case Studies

An EDTA poultice was used to remove calcareous carbonates and sulfate salts from underwater ceramics [64,73]. The combination of Na₂EDTA with sodium bicarbonate and carboxymethyl cellulose gel proved more efficient than EDTA alone for calcareous encrustations [12].

4.3. Biological Cleaning

4.3.1. Agents Used

Biological cleaning method employs biological agents, such as bacteria or enzymes to remove unwanted deposits. As an eco-friendly and safe process, biocleaning is increasingly utilized in the preservation of cultural heritage. This process is based on biological reactions or hydrolytic enzymes that are optimized under artificial conditions. Specific microorganisms, selected for their metabolic activities, are employed to clean surfaces effectively [13,42,74,75]. For example, nitrate-reducing aerobic and anaerobic bacteria, such as Pseudomonas denitrificans, Pseudomonas stutzeri, Pseudomonas aeruginosa, Pseudomonas pseudoalcaligenes and Paracoccus denitrificans, were used to remove nitrate crusts and nitrate salt efflorescence [76,77]. Among these methodologies, sulfate-reducing bacteria and bioactive molecules with hydrolytic activity have been developed. These approaches have proven to be selective and safer cleaning methods, particularly effective in removing black crusts from stone surfaces and organic materials such as glue and adhesives. [78]. Combined biological and chemical methods have proven more efficient for sulfate removal from stone artworks, with notable reduction in treatment duration [79,80].

4.3.2. Biocides and Bioformulations

Additionally, the biocide Biotin R (2%), composed of iodopropynyl butylcarbamate, n-octyl-isothiazolone, and 2(2-butoxy-ethosy ethanol), diluted in ethanol, has been successful applied to remove the efflorescence of bacteria on ceramics from archeological excavations [81]. However, despite their effectiveness as biocides, isothiazolinones are strong sensitizers, causing skin irritations and allergies, and may pose ecotoxicological hazards. In addition, this biocide is prone to excessive leaching, which could be avoid by encapsulation, offering a potentially more environmentally friendly alterative [82,83].

Bioformulations containing Desulfovibrio sp., D. vulgaris subsp. and Pseudomonas sp. Cells have been performed to remove black crusts, nitrates, animal glue residues, and salt efflorescence on ceramic material [77,84,85,86]. Furthermore, nitrates from ancient brick-works have been effectively cleaned by sepiolite as delivery system colonized by Ps. stutzeri GB94 cells [76].

4.3.3. Enzyme Applications

The removal of acrylic marker pen inks has been effectively achieved using lipases from bacterial and fungal sources, dispersed in aqueous systems and microemulsions. This method has been particularly effective on unglazed ceramic substrates, especially when the samples are unaged. Therefore, it is recommended to perform as soon as possible to maximize effectiveness [87].

4.3.4. Limitations and Considerations

While the field of biocleaning demonstrates immense potential in cultural heritage preservation, few studies have focused specifically on archeological ceramics. These treatments require carefully controlled conditions, including temperature, pH, humidity, and duration, to maximize effectiveness and avoid any negative effects. Continuous monitoring is essential to prevent excessive biological activity that could damage the object. Since microorganisms are generally regarded as hazardous agents, demonstrating the safety of bio-cleaning treatments is a crucial step for the adoption of biocleaning methods for archeological ceramics.

4.4. Clay-Based Gels

Synthetic (i.e., laponite) and natural clay minerals, such as montmorillonite, sepiolite and palygorskite are known for their adsorption and absorption properties. They can effectively trap and hold dirt, salts, and other contaminants from the ceramic surface without causing damage. The use of natural and synthetic clays in the cleaning of archeological ceramics is scarcely addressed in the literature compared to other archeological materials such as marble, stone, and metals [33,34,75,86]. The predominant clays utilized are bentonite or rich-montmorillonite clay, sepiolite, and synthetic clays as laponite.

Bentonite or rich-montmorillonite clay is often used as poultices in combination with water or with solvents, to from a gentle cleaning agent. Especially, oxalic acid is used to prepare poultices of bentonite to remove iron stains on ceramics [35,39].

4.4.1. Sepiolite

Sepiolite is a natural hydrated magnesium trisilicate that have high specific surface area (SSA) values of 77 and 399 m² g⁻¹ due to the small size of its particles, fibrous morphology and intra-crystalline tunnels [88]. This high SSA value gives a high adsorption capacity to sepiolite, which explains its use in cleaning.

Sepiolite paste, prepared by mixing sepiolite and deionized water, was used to remove stains caused by the absorption of foodstuffs or beverages on porous ceramic bodies. The paste was applied to the surfaces of the ceramic bodies, previously soaked in deionized water, on to a depth of about 1 cm [89]. Poultices of sepiolite paste of 1–2 cm thick were effectively used to clean the inner remains of organic stains of a greasy or waxy nature on pottery [7]. Poultices of sepiolite prepared with 0.25 M oxalic acid were effectively applied to remove iron stains (Figure 3) from Buncheong ware with pronounced crazing and unglazed white ware [35].

Sepiolite has also the ability to offer the anaerobic conditions required during treatment by the microorganisms like Desulfovibrio sp. involved in the biocleaning process [76]. Sepiolite was used for the removal of nitrates using Pseudomonas stutzeri on brickwords in laboratory conditions [76]. The sepiolite powder (50–70%) was mixed with a suspension of microorganisms and water under anaerobic conditions, allowing the microorganisms (bacteria) to colonize the sepiolite for 10–14 days. Once colonized, the sepiolite was ready to be applied over Japanese paper to clean the surface [85]. Japanese paper is used to facilitate the removal of the delivery system and minimizes its penetration into the pores and cavities of the original surface, reducing residue on the cleaned surface. However, sepiolite requires a lengthy bacterial colonization period before application and may experience rapid water loss after application, leading to bacterial inactivity [76].

4.4.2. Laponite

Laponite is a mix of synthetic silicates, fundamentally close to the natural clay mineral hectorite [Si₈Mg_5.34 Li_0.66 (Ca, Na)_0.66], a tri-octahedral sheet silicate. Laponite RD, the standard grade of laponite, has been widely used in ceramic conservation as effective poulticing material for the removal of stains. Laponite gel is made by mixing 3–5% laponite with distilled water that can be applied directly onto the surface of a stained ceramic body pre-soaked in water [90]. The effectiveness of this stain removal process is dependent on the drying rate of the surface gel as well as its concentration, the porosity of the ceramic body and the nature of the staining substance [91]. Laponite gel has been difficult to clean off the porous edges of the glazed Japanese ceramic and did not provide enough contact with an interleaving layer [67]. Laponite gel can causes damage due to the presence of residual gel on treated surfaces and possible diffusion of ions between a gel coating and ceramic body during treatment [91]. If used with a barrier tissue, Laponite gels can be effective at strongly pulling stains from a body, especially those held within cracks, without presenting challenges in the removal of the dried poultice [65].

Laponite is also used as a poultice in association with bacteria to solubilize phosphates, sulfates, carbonates and aged proteinaceous matter. Laponite gel allows humidifying and softening the material solubilized through metabolic activities of bacteria [41,92].

Clays are inexpensive, non-toxic, and effective in removing stains, particularly those trapped in cracks. However, despite their effectiveness, clay-based gels can leave residues or be difficult to remove when fully dried on porous surfaces like ceramics [91]. To address this, using poultices with a barrier tissue such as Japanese paper can facilitate the removal of the dried poultice.

4.5. Organic Gels

Gels are biphasic systems consisting of a polymer and a fluid phase. The polymer forms a three-dimensional network in the liquid, trapping the fluid and limiting evaporation and release [93]. Interactions between polymer chains, which can be reversible (e.g., hydrogen bonds, van der Waals forces) or irreversible (covalent bonds), determine the gel’s properties. Gel-based cleaning protocols are effective due to their high retention and rheological properties, enabling precise and selective treatment of the surface layer. Gels can adsorb unwanted substances, such as corrosion products and degraded protective materials, into the polymer matrix. Additionally, they significantly reduce solvent fumes, creating a safer work environment for operators.

4.5.1. Agar

Agar, a complex polysaccharide derived from red-purple algae of Rhodophyceae, typically from Gelidium and Gracilaria genera, consists of agarose and agaropectine. It is readily soluble in hot water, stable in both alkaline and acidic conditions, and is a safe, non-toxic, and eco-friendly material [94,95]. It has been effectively utilized, as an aqueous gel or poultice, for cleaning various art forms (mural painting, wood, stone, plaster, paper, and textiles) due to its ability to control water release and it gentle interaction with substrate [31,32,72,95,96,97,98,99,100,101]. The microstructure of agar is characterized by a significant number of pores with a uniform size, which enhance its ability to retain water and water-soluble dissolved substances. Capillary forces facilitate the transfer of material from the substrate into the gel, thereby minimizing the need for post-treatment cleaning of surfaces and suggesting its potential for salt extraction [102]. Studies evidenced that the application of agar on ceramics leaves no residue within the porous materials (Figure 4D–F) [95]. However, the surface porosity and topography increase the amount of residues that are difficult to remove using standard cleaning procedures [103].

Agar-based gels incorporating ethanol or NaOH have to be effective for removing organic material such as shellac, gouache, acrylic emulsion, and PVA on ceramic surface without causing damage. For the cleaning of heavily soiled substrates, the gel should be reapplied intermittently to avoid saturation of the gel and diffusion of the solubilized material back into the substrate [95].

Agar is also employed as a thickening agent in chemical solutions to reduce the evaporation rate of cleaning products. This property minimizes diffusion and enhances control over the treated area [56,71,104]. Furthermore, agar is often used as a support medium for the growth of microorganisms, such as bacteria capable of degrading contaminants [81].

However, several challenges must be addressed. These include the necessity of mixing agar gels with water, difficulties in incorporating certain solvents, limitations on the use of surfactant, and potential risks of biological deterioration. Another limitation is the dissociation and liquefaction of the gel caused by the disruption of hydrogen bonds in the presence of surfactants. Despite these drawbacks, agar-based gels show significant potential for conservation treatments. Nevertheless, further research is needed to evaluate issues such as potential residue formation and interactions with different substrates before they can be widely adopted on various materials.

4.5.2. Cellulose Powder

Arbocel^®, a microcrystalline cellulose derived from natural cellulose fibers, is widely used in cleaning applications due to its high absorbency, non-abrasive nature, chemical stability, and ease of removal. These properties make it an effective support material for poultices used in extracting soluble salts and cleaning stone, marble, and ceramic surfaces [34,39,105,106,107]. The uniform pore distribution of Arbocel^® poultices makes them more suitable for salt extraction on substrates with medium to coarse pores (≥15 μm), although their efficiency diminish on finer-pored substrates [107]. Additionally, Arbocel^® poultices have been successfully combined with EDTA, acetic acid, and nitric acid for the removal of calcium carbonate deposits from archeological ceramics [15,39]. Arbocel fibers saturated with a Triton-based microemulsion was used to clean pottery excavated from Athribis, Egypt, which exhibited various forms of deterioration, including crystallized salts, soiling, muddy layers, and adhered black soot. The poultice successfully removed all surface deposits within 2 h (Figure 4A–C and prevented the formation of light surface encrustations [12]. Arbocel^® is also used for biocleaning preparation by mixing it with a biomass suspension to obtain a homogeneous mixture of the desired density. The system is applied to the surface over Japanese paper previously moistened with a phosphate buffer [80,84].

Similarly, fibrous cellulose powder derived from high-quality cotton linters has been employed in poultices mixed with deionized water for the removal of water-soluble stains from Japanese glazed ceramic jars [67]. However, due to the heterogeneous moisture transfer inherent to the fibrous structure of cellulose, the release of the cleaning solution can be uneven, which may result in less homogeneous cleaning effects across the surface. These limitations have been observed in comparative studies, which show that cellulose-based poultices sometimes require multiple applications to achieve thorough stain removal, and that their performance can be less uniform compared to synthetic adsorbents [66].

4.5.3. Xanthan Gum

Xanthan gum (Vanzan^® NF-C) is a high molecular weight, water soluble polysaccharide widely used as a gelling agent in the cleaning of cultural heritage objects [108,109,110,111]. Its ability to form a gel that adheres to surfaces allows for effective removal of unwanted material. Additionally, its non-toxic and biodegradable nature makes it a safe and environmentally friendly option for conservation practices. Xanthan gum is especially effective in aqueous media and can tolerate a broad range of pH values and ionic strengths while maintaining high viscosity [108]. It is also used as a thickening agent to prepare acid gels or pastes for the removal of carbonates from ceramics [39]. Furthermore, a rigid xanthan gum gel, combined with EDTA, acetic acid, and nitric acid, has been used to remove calcium carbonate deposits from archeological ceramics. However, it was found to be less effective than Arbocel^®, likely due to insufficient time for the gel to interact with the calcareous deposits [15].

The ability of gels to control solvent evaporation rates, surface contact time, and minimize human exposure to toxic solvents has made them a preferred choice for conservators. By incorporating enzymes and various surfactants into a gel system, cleaning efficiency can be further enhanced. However, it is important to note that, depending on the nature of the ceramic material and the contaminant, combining a gel system with other cleaning methods may be necessary to achieve satisfactory results. Despite the advantages, challenges persist, particularly regarding the long-term effects of gels under different environmental conditions, which could lead to alteration, especially on painted ceramic surfaces. In addition, issues such as the potential for gel residues, incompatibility with certain conservation products, and the risk of undesired interactions should be considered. The use of barrier materials, such as Japanese paper, can help prevent residue deposition and facilitate gel removal, mitigating some of these concerns.

4.6. Resins

4.6.1. Resin Types and Their Efficacy

Resins are used in cleaning for their ability to adhere to contaminants, while their chemical stability ensures that the ceramic material remains unaltered during the process. Among these resins, Styrene-divinylbenzene copolymer resins with sulfonate functional groups are widely used in various fields, including ceramic cleaning. Notably, Amberlite XAD^® (Rohm and Haas Company, PA, USA), Amberlite IR 120 H, and Ionex H^® (DuPont, Wilmington, DE, USA) are prominent. Amberlite resin effectively removes calcareous deposits from glazed ceramics when applied to the surface decoration layer under controlled conditions (30 °C and 100% relative humidity), preventing damage to the ceramic body [9]. It has also been used in agar-agar gel at 40 °C to clean the glazed ceramic, with repeated applications potentially needed for stubborn incrustations [56]. Amberlite IR 120 H, a strong cation exchange resin, when mixed with deionized water (1/1), efficiently eliminates encrustations from pottery fragments recovered during underwater excavations in southern Italy, preserving the substrate intact [1]. Amberlite XAD 7HP^® polymeric adsorbent resin has been shown to be more effective than cellulose pulp, cellulose powder, and Laponite^® (BYK-Chemie GmbH, Wesel, Germany) gel in reducing stain particles on ceramics [66].

4.6.2. Limitations and Recommendations

The carbonate deposits on painted ceramics from Ibero-Roman remains (7 BCE-5 CE) at the site of Los Villares de Andújar (Jaén, Spain) were cleaned using an ion exchange resin (IONEX H CTS). Applied directly to the ceramic surface for 20 to 30 min at 90% w/v in demineralized water, the resin demonstrated effectiveness. However, both macroscopic and microscopic observations indicated that the resin caused damage to the painted surfaces, despite the polychrome layer exhibiting high cohesion and adhesion to the ceramic [63].

The sulfonate functional group provides strong acidic properties to resin, enhancing their efficiency in ion exchange processes. However, this acidity can affect the treated surfaces, underscoring the importance of carefully controlling contact times during cleaning operations. To address these challenges, future research should focus on the development of modified resins or buffer systems that neutralize acidic effects and minimize surface damage. Additionally, studying the long-term interactions of residual resins with ceramic necessary to ensure safe application. Comparative studies of wide range of ceramics, including variations in conservation states, porosities, glazes, and contaminant types, could further refine best practices for resin use. Finally, integrating resins with complementary cleaning systems, such as enzyme-based gels or surfactants, could potentially enhance cleaning efficacy while minimizing associated risks.

4.7. Laser Cleaning

4.7.1. Principles and Operating Parameters

Laser cleaning, established in the 1980s for heritage conservation, is a sensitive and irreversible process that requires careful selection and optimization of laser parameters. It employs a focused laser beam to remove unwanted layers (e.g., dirt, crusts, corrosion) from a surface. The process relies on the absorption of laser energy by the contaminants, causing their ablation without damaging the materiel treated. Key parameters like wavelength, pulse duration, and energy intensity are adjusted to suit the substrate and unwanted material. Understanding the ablation mechanisms and potential side effects is crucial for successful cleaning [112,113]. Laser cleaning offers precise, minimally invasive treatment for materials, focusing on surface layers with controlled removal of encrustations. It allows high selectivity making it a less intrusive and more easily controlled method compared to traditional techniques. Additionally, it avoids many drawbacks of conventional cleaning processes, such as multi-step treatments and the sue of solvent [114,115].

4.7.2. Types of Lasers and Mechanisms of Action

Various types of lasers are employed in artifacts cleaning of, including Erbium:Yttrium-Aluminum-Garnet (Er:YAG), Neodymium-doped Yttrium Aluminum Garnet (Nd:YAG), and Femtosecond lasers. Femtosecond lasers, with their short pulses, can remove contaminants while preserving the integrity of the substrate. The selection of laser type and parameters depends on the specific materials and contaminants involved [116].

The Nd:YAG laser, operating at 1064 nm, was employed effectively to eliminate insoluble aluminosilicate crusts from ceramic from eastern Crete, with energy fluences between 4 and 8 J/cm². This method successfully eliminated the crust without causing discoloration or surface irregularities, especially when combined with water application during cleaning [117]. The Nd:YAG laser has also been used to clean Roman mosaic fragments, targeting white tesserae to highlight contrasting patterns of the mosaic [101]. Furthermore, Nd:YAG laser at energy densities of 1.60–1.90 J/cm² and 10 ns pulse durations, effectively removed fungal layers from ceramics found near Durankulak, Bulgaria [22]. For cleaning archeological pottery and glass, Nd:YAG laser radiation at 1.064 and 532 nm wavelengths with 100–500 mJ cm⁻² was effective in removing strong sinter layers [21]. However, at higher fluences or with picosecond pulses (1–20 J/cm², 1064 and 532 nm), thermal effects such as local melting of ceramic glazes and microcracking were observed in archeological ceramics from Yazd, Iran [23], as well as 18th-century Portuguese tiles [118].

The Er:YAG laser, with a wavelength of 2.94 μm, is particularly effective for ceramics cleaning, as it is highly absorbed by –OH and –NH bonds in molecules, making it well-suited for removing aged resins, biological growth, and adhesives from ceramics [119,120,121,122] (Figure 5). In some cases, pre-wetting with solvents can enhance laser absorption for surface layers with low O–H content [121]. The Er:YAG laser has been successfully used to remove epoxy resins from terracotta and an old cellulose nitrate adhesive from Greek ceramic vase fragments [120,123]. At an energy of 3.5 J/cm², it transforms adhesive into a soft white mass, making it easy to detach from the ceramic substrate without causing material loss [120].

4.7.3. Risks and Limitations

Laser cleaning combined with mechanical methods can improve efficiency and speed [117]. However, Nd:YAG and Er:YAG lasers can sometimes be insufficient, as they may not completely remove deposits and could cause color changes [123]. The Nd:YAG laser, in particular, presents challenges when used on polychrome and can be cost-prohibitive for large surface areas [124].

Recently, Pulsed Laser Cleaning (PLC) has been developed as an environmentally friendly method with minimal thermal impact, effectively removing contaminants such as oxidation layers, dust, organic matter, and other unwanted deposits from ceramics. This technique is particularly effective in preserving the integrity of the underlying material while ensuring a thorough cleaning process [25].

Laser cleaning is a precise, non-contact method that minimizes the risk of damage to delicate surfaces by targeting contaminants based on their optical properties, leaving underlying materials intact. It is environmentally sustainable, avoiding chemicals and abrasives, and is effective on archeological ceramics conservation. However, the high cost of laser systems limits access for smaller institutions. Laser cleaning, while advanced and precise, is thermally sensitive and requires skilled operators. Safe and effective application depends on a thorough understanding of material–laser interactions and careful adjustment of laser parameters, as improper settings can cause discoloration, microcracks, melting, or material loss, particularly on fragile surfaces. It is most effective for superficial contaminants and may not remove deeply embedded layers, requiring skilled operators to adjust parameters for safe and effective cleaning.

4.8. Plasma Cleaning

4.8.1. Principles and Advantages

Plasma cleaning is an emerging technology that provides an efficient, non-invasive solution for surface cleaning, offering the advantage of leaving no residues or contamination. This process utilizes ionized gases (plasma) to clean and modify surfaces. Plasma is generated by applying an electric field to a gas, causing its molecules to become ionized. The ions and radicals produced interact with contaminants on the surface, breaking them down and removing them. Plasma cleaning employs energetic plasma or dielectric barrier discharge plasma, created from argon, oxygen, air, and hydrogen/nitrogen, effectively removing impurities and contaminants such as invisible oil films, dust, and other contaminants [30].

Low-temperature plasmas are particularly advantageous in that they do not require direct contact with the surface, reducing the risk of mechanical damage. Furthermore, plasma cleaning does require harmful solvents, making it an environmentally friendly alternative to traditional cleaning methods [26,27,29]. It is applicable to a broad range of materials, including natural polymers, metals, ceramics, and stones, and is effective even on objects and materials with complex surface geometries [29]. However, hydrogen-based low-temperature plasmas are not suitable for objects with a fragile structure, such as glass or ceramics with breaks, and can be less effective than traditional cleaning methods in certain cases [28].

4.8.2. Applications and Limitations

Radio frequency (RF) hydrogen (H₂) plasma, a glow discharge typically produced using gases such as oxygen and hydrogen, has demonstrated effectiveness in removing metallic stains from brick surfaces, such as those found at Prince Yousef Kamal Palace, where it reduced. reducing the darkening and thickness accumulation of both iron and copper on aged samples [11].

Overall, despite its potential as a gentle and effective cleaning method, the application of plasma cleaning for archeological ceramics remains limited. This limitation is due to the lack of studies evaluating its safety and effectiveness on fragile and porous ceramic materials, as most research has been conducted on test samples or relatively well-preserved historical ceramics.

5. Considerations in the Cleaning of Archeological Ceramics

The cleaning of archeological ceramics involves a complex process requiring both technical expertise and a deep understanding of the artifact’s historical and environmental context. It is particularly critical for low-fired or degraded ceramics, which may have been affected by manufacturing methods, burial conditions, and excavation processes. Cleaning is intended not only to remove surface deposits, stains, and encrustations but also to protect the ceramic from adverse reactions with its environment. However, cleaning should not be performed simply because it is feasible. Instead, it must follow a comprehensive evaluation of the artifact’s condition, incorporating its historical, cultural, and material significance [7]. Decision-making in the cleaning of archeological ceramics requires a careful and interdisciplinary approach to balance the removal of deposits with the preservation of embedded historical and cultural information. The process must be guided by a detailed assessment of the artifact’s condition and its archeological context. Particular attention must be given to specific types of residues that may contain valuable archeological information, such as soot deposits on the inner surfaces of vessels, mineral or organic sediments resulting from ancient liquids (e.g., wine, oil, beer), or resinous coatings intentionally applied as waterproofing agents. These materials should not be removed before appropriate analytical characterization, as cleaning with detergents or organic solvents can irreversibly damage or eliminate such evidence [125,126,127]. Physicochemical analyses (i.e., chromatography, Fourier Transform Infrared spectroscopy, Raman spectroscopy) should be systematically employed to distinguish harmful contaminants from archeologically significant residues and to guide cleaning strategies accordingly [127,128]. In some cases, layers of corrosion or mineral-preserved organic residues may carry valuable archeological information, such as insights into past diets, trade, or environmental conditions. Such layers should be preserved unless they are unstable, obscure essential details, or compromise the artifact’s visual or interpretative integrity [129].

Conservation extends beyond technical cleaning procedures; it is a collaborative effort involving archeologists, conservators, and scientists. Together, they aim to safeguard the artifact’s archeological significance while enabling its study and preservation. Effective cleaning strategies ensure that any interventions do not compromise the artifact’s long-term preservation or its contextual value [130]. Recent advancements in cleaning technologies, combined with molecular-scale studies of treatment impacts, highlight the growing scientific rigor in conservation practices. These developments underscore the importance of integrating conservation perspectives into archeological research. A collaborative, interdisciplinary approach combining new cleaning technologies with ethical conservation practices ensures effective outcomes that support both preservation and archeological research goals.

Practical guidance for cleaning archeological ceramics includes the following instructions:

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Prioritize the preservation of layers that may carry archeological information unless they are unstable, obscure essential features, or compromise interpretative integrity.

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Conduct thorough preliminary analysis of both the artifact and contaminants before any intervention.

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Select cleaning techniques according to ceramic material, deposit type, and structural stability, balancing efficacy and risk.

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Adjust treatment parameters carefully (e.g., energy, solvent concentration, exposure time) to minimize damage.

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Carry out analytical assessments of the treatment’s impact to ensure safe and effective outcomes.

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Document all interventions, including methods and materials used, to support reproducibility and future research.

6. Conclusions

Careful cleaning of archeological ceramics is essential to preserve the historical and cultural information they hold. The diverse conditions of burial and excavation often lead to varied surface deterioration and contamination, requiring adaptable methods that respect the fragile and unique nature of these artifacts.

This review has demonstrated that no single cleaning technique is universally applicable to all archeological ceramics. Each method, including chemical solutions like EDTA salts, natural and synthetic clay minerals, resins, and advanced technologies such as biocleaning, plasma, and pulsed laser cleaning, offers distinct advantages and faces specific limitations. The choice of product and application method should be guided by the characteristics of the sherd, including porosity, fragility, type of surface decoration, and the nature of the deposits or contaminants. For instance, while chemical solutions can efficiently remove mineral deposits, they require careful monitoring to prevent damage. Similarly, natural clays and gels offer eco-friendly and non-invasive alternatives but may require repeated applications for heavily encrusted surfaces. Advanced methods, such as plasma and laser cleaning, are promising but need further refinement to address their limitations, particularly on fragile or painted surfaces.

Based on the material properties and condition of the ceramics, a practical selection guideline can be proposed: controlled, low-impact methods (e.g., gels, natural clays) should be prioritized for fragile or decorated sherds, chemical treatments can be applied for mineral encrustations with careful monitoring, and advanced methods (plasma, laser) are recommended only when conventional techniques are insufficient or when high precision is required.

Through this comparative analysis, the importance of tailoring cleaning approaches to the specific needs of each artifact has been underscored. This review not only informed the author’s approach to the conservation of archeological ceramics but also aims to serve as a practical guide for other conservators in decision-making regarding method selection and sequence of application. It emphasizes the value of combining traditional and innovative techniques while considering the balance between efficacy and preservation.

Future research should focus on developing cost-effective, sustainable, and non-invasive cleaning methods that address the complexities of archeological ceramics. Priorities include studying long-term effects, improving compatibility with fragile materials, and advancing eco-friendly alternatives to safeguard this archeological heritage.