| Abstract: | SUMMARIES. The widespread occurrence of thecotrichite, in the museum environment is explained theoretically by construction and examination of its phase diagram. Thecotrichite formation was simulated in the laboratory to identify the key factors involved in its production. This efflorescence occurs on porous limestone or calcareous artefacts such as pottery, stored in wooden cabinets that generate acetic acid vapour. Salt production depends on the moisture content of the object and the concentration of acetic acid in its surroundings. Furthermore, for thecotrichite to form the artefact must contain soluble chloride and nitrate salts.
CONCLUSION. These results confirm that objects are prone to theco¬trichite production when they are comprised of or con¬tain calcium carbonate contaminated with chloride and nitrate ions and, furthermore, are located in an environ¬ment containing acetic acid vapour. In addition, it has been demonstrated that the ambient RH is also an important secondary issue in the formation of theco¬trichite; low values of RH inhibit efflorescence growth. While chloride and nitrate salt residues are an essential prerequisite for the formation of thecotrichite, salt con¬taminants in general predispose artefacts to acetate efflorescence formation. Other studies have demon¬strated a positive correlation between efflorescence formation and salts present in the affected item [20, 21]. These salts occur as residues from previous conditions or have been introduced by conservation or cleaning treatments [12, 20—23]. The reasons for the deterioration of some items held in the same environment as others that remain in pristine condition warrant further study but the presence of salt residues is undoubtedly an important factor. Thecotrichite efflorescence most usually takes the form of whiskery crystals. This is in line with previous observations that the formation of efflorescence whiskers occurs on porous substrates without any liquid water supply from the interior (see for example [24]). Thus, the appearance of thecotrichite on artefacts is consistent with whisker growth from a small amount of solution in the zone of efflorescence. When further solution is supplied and conditions for whisker growth are maintained, the process continues and a dense carpet of thecotrichite is formed. The question remains: how can acetate efflorescence formation be prevented on porous museum artefacts? The standard treatment method for obviating salt efflorescence is desalination. If this approach is to be followed to prevent the formation of thecotrichite, care must be taken to ensure that salts are removed completely from the core of the object otherwise they will be drawn to the surface by capillary action and become available for reaction with acetic acid vapour. Perhaps the most straightforward suggestion would be to remove susceptible items from acetic acid contaminated storage or display areas. However, if whole collections are affected, this solution becomes impractical. Testing of materials to ensure suitability for showcases has been advocated since the 1970s [25] and later publications demonstrate the vast range of materials tested for emission of acids and aldehydes (see, for example, [26-28]). As demonstrated by the phase diagram, even with small quantities of acetate present in the solution phase, the stability field of thecotrichite will be reached. Since the concentration of acetate ions present in the solution phase is determined by the acetic acid concentration and the humidity of the surrounding environment, con¬sideration can usefully be given to the reduction of both these factors. In this 18-month study, no thecotrichite formed on limestone blocks held at an acetic acid vapour concentration of 10 mg-m-3 at 75 or 33% RH. Similar studies have shown that lowering the RH to below 30% retards acetic-acid-induced degradation of shells [29] and lead [30]. It is important, however, not to misinterpret the benefit of low acetic acid concentrations as 'threshold' values. If the limestone blocks had been left in the low acid-contaminated environments for a longer period of time, it is highly probable that thecotrichite would have been formed. In practice, acetate efflorescences have been observed for many items over a wide range of contaminated environments with varying acid concen¬tration [9, 31]. Unfortunately there is no easy, global, solution. Each situation must be assessed on its own merits to determine the best practical answer to the problem. Careful consideration must be undertaken of the artefact's composition, the environmental conditions (pollutant concentration and RH) under which the object is stored and possible soluble salt contamination. Finally, the authors realize that in real situations ions other than calcium, chloride, nitrate and acetate will also be present in solutions found in the pores of contamin¬ated objects. Although this study focused solely on the thecotrichite system, it should be noted that the far more complex quinary system, including sodium in the phase diagram, is currently under consideration. Although the phase diagram is not yet complete it has been observed that the main conclusions are also valid in the quinary system. Addition of a relatively small amount of acetate to a solution containing calcium, sodium, chloride and nitrate ions will lead to the formation of thecotrichite. |
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