Porous plasters

The use of porous formers ia the dippiag process, or porous molds prepared from plaster of Paris or uaglazed porcelaia with a surface pore size smaller than the majority of mbber particles, has been widely adopted ia the latex iadustry. With the porous porcelaia formers, the mbber particles are filtered oa the surface of the formers. The mbber latex coagulates because of its high coaceatratioa to form a film of increa sing thickness as more water is absorbed iato the ceramic. Its rate of iacrease diminishes sharply beyoad an optimum period of time, however, depending on the various characteristics of the ceramic. 

Absorption - Processes water can be removed from a material by the capillary action of porous bodies. An example is the cream of clay and water used for casting pottery, which is deprived of the greater part of its water by placing it in molds of plaster of Paris. The capillary character of this mold withdraws the water from the liquid clay mixture and deposits upon itself a layer of solid clay, the thickness of which is controlled by the time of standing. Certain types of candies, such as gumdrops, are dried mainly by contact with the starch molds in which they are cast. The drying effect of sponges, towels and materials of this kind is due to this same action. 

Any small amount of vapour which might enter through faults in the vapour harrier should he encouraged to pass through the inner (cold side) skin of the structure to the coil, rather than he trapped within the insulation. It follows that, if the vapour harrier is at all suspect, the inner wall coating should he more porous. In traditional construction, this was provided hy an inner lining of cement plaster or asbestos cement sheet, hoth of which transmit vapour. The modern use of impervious materials on hoth skins requires meticulous attention to the sealing of any joints. 

Procedure in ceramic processing whereby slip is contained in a porous plaster mould prior... 

This manipulation requires that the mass to he formed be of sufficient tractability to be easily introduced into the cavities of the mould by kneading, and that, when pressed, It be of sufficient tenacity as not to bend or yield upon removal. It must, therefore, acquire consistency, and dry in the mould—a condition which Tenders it absolutely necessary that the moulds he constructed of substances of a porous nature, otherwise the pieces would adhere and lose their shape upon removal. The moulds are, therefore, generally formed of gypsum—plaster of Paris—and sometimes of burned clay, and are of every variety of size and pattern, a complete set of pattarns being required for evory new design, and for every size of the same pattern. Moulds for plates, dishes, and other shallow articles, consist of only one pioco, while for jugs, vases, and more elaborate forms, several pieces or moulds are employed. 

Porous surfaces, such as wood, plaster, and concrete, are also often sampled with wipes. Surface sampling of porous materials, however, does not provide any information on the contaminants that may be trapped inside the pores, cracks, and seams of the sampled surface. A better way to characterize porous materials is to collect and analyze chip samples. 

E.g., inorganic materials (ceramics, glass, porcelain) can be waterproofed with easily hydrolysed alkylchlorosilanes (methyltrichlorosilane, dimethyldichlorosilane, ethyltrichlorosilane, diethyldichlorosilane). Metals and porous materials (paper, leather, textile, plaster, cement, gypsum, etc.) should not be waterproofed with alkylchlorosilanes, because they release hydrogen chloride, which destroys these materials. Alkylchlorosilanes can be successfully replaced with silicone oligomers with aminogroups or hydrogen atoms in the molecule. 

One of the methods for further processing of such paste pieces is pressing in plaster moulds. The mould is porous so that it releases compressed air and simultaneously absorbs some of the water from the mix. The technology is inexpensive and used in particular with small, simple articles where low moulding pressures are required. 

Stabilisation by climate control. The most desirable method to prevent damage induced by the repeated cycles of crystallisation and hydration would probably be environmental control. However, neither the selection nor the maintenance of such an ideal environment is possible if looked at realistically. Predictions of salt crystallisation and hydration from mixed salt solutions are more or less impossible, taking into account all the different parameters that influence the process. Sawdy provides a brilliant overview of the subject and considers relative humidity, temperature, air movement, type and structure of the porous support, salt mixture composition and salt concentration. It is necessary to consider not only consolidation treatments of the plaster or the paint layer, which as such may influence the transition behaviour of the salts, but also the influence of microbial extracellular slimes on the porosity of the system. 

The situation becomes even more unpredictable since crystallisation of even pure salt solutions does not occur at a given point of relative humidity. Also the behaviour of the solvent, in this case water, within the porous system of the multi-layered plaster and paint layer system is different to, and much more complicated than, in vitro experiments. Therefore, we have to accept that the prediction of crystallisation and hydration cycles of salt mixtures within the porous wall painting system is one of the most difficult tasks one can imagine. 

---