materials like dispersed white lime, contain minor organic ingredients to keep
them dispersed or to stabilise the product, and these may serve as organic
substrates for microbial infestation. In these circumstances, one then has to
consider the risk of biodeterioration.
4.1 Consolidation
Consolidation generally can be divided into two types of methods: inorganic or
organic consolidants. The latter may be natural organic polymers or even syn-
thetic polymers. Both types of treatment may cause longer-term drastic alterations
although acceptable results may have been obtained directly after application.
Consolidation by inorganic materials. Up to now, a mineral treatment such
as the application of barium hydroxide (described earlier) seems to be the
only method that does not turn out to be detrimental in the end, as long as it
is applied carefully and under the right conditions.
Application of “Wasserglas” (sodium silicate) as a consolidant for wall paint-
ingscame into use at the beginning of the nineteenth century. It was used for
consolidation of stone monuments, plaster, lime renderings and also as a bind-
ing for silicate paints. Soon after, however, failure of the application became
common and the formation of crusts was often seen, leading to enhanced dete-
rioration by flaking. When used in combination with Portland cement, tremen-
dous amounts of sodium carbonate and potassium carbonate are formed.
Although potassium carbonate is hygroscopic in itself, and should not crys-
tallise in solution, it may form dark spots on the paintings. Moreover, in paint-
ings with autochthonous salts, the transformation to less hygroscopic salts
may be induced and this lead to efflorescences.
In cases of high humidity and known infestation by heterotrophic microor-
ganisms, consolidation treatments based on mineral, rather than organic
components, are more appropriate. Therefore consolidation by silicate-based
materials was introduced (methyl or ethyl silicate). This led to human health
risks. Besides the known risk of silicate formation on the cornea, that may lead
to blindness due to methyl silicate, both forms may irritate the respiration
system. Moreover, laboratory investigations as well as observations at treated
sites, showed an unexpected intensive growth of fungi on ethyl silicate
(Figure 8). This is difficult to explain since the catalysts within the products
belong mainly to the group of organo-tin compounds, known to be very
potent biocides. Until now, the cause of these findings cannot be explained.
Consolidation by organic materials. More commonly, organic polymers
have been applied for consolidation of paint layers. Natural polymers were
most frequently applied in Europe, including animal glue, casein, egg yolk
256 Chapter 10