Prékopa Ágnes (szerk.): Ars Decorativa 31. (Budapest, 2017)
Kornélia HAJTÓ: Restoration Options for Architectural Ceramics
dioxide. The silicon dioxide makes chemical bonds with the original silicate material, forming quartz bridges that greatly increase the lifetime and load bearing capacity of the object. The process may be represented by the simplified equation: Si (OEt)4 + 2H20 h> Si02 + 4EtOH tetraethyl orthosilicate The alcohol produced in the reaction evaporates and the silicon dioxide hardens the material. Most construction materials (stone, brick, mortar, rendering, concrete and architectural ceramics) are made up of complex silicon-containing compounds, so that the strengthening provided by this kind of conservation agent effectively comes from the same material as the porous surface it permeates. Agents of this kind have many advantages. One is that the small molecular size of the components and the low viscosity and surface tension of the solvent means that they quickly penetrate deep into many porous materials. They have no by-products, because the alcohol evaporates without trace. After solidification, the treated material retains its porosity and does not become hydrophobic. The SiO, which forms is also highly resistant to UV radiation. Silicic acid ester hardening can greatly reinforce the micro-structure of the material being treated. Its small molecules, however, make it unsuited for repairs of cracks and fissures. The reaction causing the hardening is slow and has to be accelerated with catalysts. The catalyst also affects the bonding strength of the Si02 and thus the effectiveness of the treatment. The agent consists of silicic acid ester, catalyst, water, and a solvent carrier (because the first component is not soluble in water). If the Si02 forms too quickly, the strengthening effect is diminished; if too slowly, the agent can evaporate and accumulate in the layers near the surface, and create a crust. Because of the variability of the components, there is a wide range of silicic acid ester agents on the market (Wacker, Remmers, Kern, Mapei). Each manufacturer offers several variations, mostly differing by the concentration of silicic acid ester in the solution. It is thus possible to choose the variant most suited to the particular parameters of the material to be strengthened. Research published in 2006 compared conservation agents of two different concentrations (100%- and 20%- silicic acid ester content) on two kinds of coarse limestone before and after impregnation.6 For effective results, it is essential to carry out prior material tests, trial treatments and impact studies. The ideal arrangement for historic building restoration is for restorers and scientists to draw up a customized treatment system for each individual problem. In addition to exterior ceramics, silicic acid ester strengthening is often a preferred process for treating artworks in museums. Adhesives There are also several different products for attaching exterior ceramics. One way of categorizing adhesives is by bonding mechanism. Synthetic resin—specifically epoxy— adhesives are widely used for fixing ceramics, and there are weather-resistant variants that have been developed for exterior use (Akemi, IMPA). They have very high adhesive strength, and consist of two components, the epoxy resin and a hardener. The resin polymerizes in an epoxy ring through 100
