JAIC 2002, Volume 41, Number 3, Article 5 (pp. 269 to 278)
JAIC online
Journal of the American Institute for Conservation
JAIC 2002, Volume 41, Number 3, Article 5 (pp. 269 to 278)




Clearly the most important results obtained are that the behavior of gypsum toward moisture take-up is, in some cases, modified by the presence of other components in the mixture and that this behavior will depend on the nature of the latter. Furthermore, the dehydration behavior of gypsum will vary depending on the amount of water vapor present in the oven—those without ventilation—from the drying of the samples. If there is no significant amount of moisture in the oven, gypsum will dehydrate to hemihydrate above 40șC and to insoluble anhydrite at 105șC. If some moisture is present at this temperature, soluble anhydrite may form, which tends to rehydrate fairly rapidly if not kept in an appropriate desiccator. As mentioned, the stability relations between these phases, hemihydrate-soluble anhydrite and anhydrite proper, are not well defined

Table . Comparison of Measured and Calculated Weight Losses at 105șC
Table . Various Analytical Methods Currently in Use for Mortar Analysis. Sample Size, Measurement Precision, and Approximate Relative Error of the Approach
(Palache et al. 1966). If a high amount of moisture is present in the oven, gypsum may only partially dehydrate to hemihydrate even at 105șC. The conditions under which analysis of gypsum-containing mortars is carried out are therefore critical.

Furthermore, several practical conclusions could also be reached. The first one is that the drying procedure may not be as fast as supposed, especially if samples contain a relatively high amount of moisture or gypsum. The usual practice suggests 24-hour drying at 105șC, weighing and redrying for another 24-hour period to confirm the stability of the weighing. Normativa Manufatti Lapidei (NORMAL) procedures just indicate the maximum allowable change in two subsequent weighings but do not even indicate an approximate length of time required to reach this value. In our experiments, this process took at least one week. This time frame is an important consideration when planning to carry out a set of mortar analyses. In particular, architectural conservators who do not routinely carry out analyses but do so only on an ad hoc basis need to take this time frame into account.

A similar effect was noticed when samples of this type were left to dehydrate in the desiccator. Given the nature of the desiccant used, the amount, exposed surface, and size of the desiccator will determine its drying efficiency. And since the indicator color change occurs only above 30% RH, it is clear that samples containing a high amount of moisture are not dried completely. Therefore the second practical recommendation is that to keep samples dry, they should be kept to a minimum, and the desiccant in dessicators needs to be changed periodically, particularly when larger “storage” cabinet dessicators are used.

Finally, the issue of the error introduced into the analysis results, depending on the drying temperature used, has to be considered. At 40șC, it has been shown that samples do not dry out completely. At higher temperatures, gypsum dehydrates in an unpredictable way when large amounts of moisture or other salts are present. Therefore, the analyst needs to consider the aim of the analysis he is performing and take into account the nature of the sample he is analyzing before deciding the temperature at which the samples should be dried.

Copyright © 2002 American Institution for Conservation of Historic & Artistic Works