many monuments in India are in a dilapidated state. Among all causes - pollution and humidity, temperature, sunlight and salt crystallisation - water does the greatest harm by transporting acids from waste gases (chemical corrosion), salts in the building material to the surface. It is also responsible for frost damages (mechanical corrosion) and growth of microorganisms (biological corrosion). "Water is the biggest enemy of materials especially monuments. Like other monuments, the Taj Mahal also suffers from these water-related problems," says S P Singh, head of the department of conservation at the National Museum in New Delhi.

Now a German-based company, Wacker-Chemie, hopes to introduce its new technology - that uses silicon polymer to strengthen monuments - on the marbles of the Taj Mahal. This polymer contains silica as its main ingredient. It works as a consolidant, impregnent and a water repellent. Heinz Geich of Wacker-Chemie said in New Delhi recently that his company would try to analyse whether this technology can help to restore the Taj and other monuments in India.

For restoring monuments, it is important to reduce the amount of water absorption by the building material. Past experience on other monuments has shown that silicon resins can be successful, says Geich. The big advantage of silicon resins lie in their similarity with minerals found in the stones that make the monuments.

Wacker-Chemie has worldwide experience in conserving and preserving famous monuments and statues such as the Brandenburg Gate in Berlin, Michaelangelo's Pieta in Rome and the Temple Site Chichen Itza in Mexico. Explaining the process, Geich says that the microstructure of the deteriorated stone is stabilised through the introduction of a binder. The products used to conserve the stone are called impregnating agents and are applied to saturate the stone. The impregnating agent can impart certain properties that the construction material either did not have or has lost during use. One of these, is water repellency. Construction materials are not usually water-repellent. It also strengthens the microstructure and as far as possible restores the stone to its original condition.

The idea for strengthening stone with ethyl silicate was first recommended by the German chemist August Wilhelm von Hofmann. Ethyl silicates are compounds formed by the reaction of silic acid with an alcohol. In lay terms, this is essentially a chemical reaction between quartz, which is often regarded as a precursor of all mineral compounds and an alcohol to give a soluble compound. The soluble compound is then applied to the stone. On penetrating the stone, it is converted back into quartz (silica). The original alcohol is released and evaporates. Other major advantages of ethyl silicates is their good penetrating power and the fact that the pores never become clogged. The open pore microstructure assures gas and water vapour permeability. Thus the stone continues to breathe.

Stone strengtheners based on ethyl silicates are generally applied by spraying or flooding. "In our experience the best working conditions are a relative humidity of 40 to 70 per cent and a surface temperature on the construction material of 10