Conservation of render and plaster

Consolidation with water glass (soluble glass)

Crazing and the characteristic pattern that develops in renders are often a result of natural processes in the render. Leaching of lime can also give porous surfaces, yet with a good adhesion to the substrate.

Natural cracking processes in lime renders

The characteristic pattern often found in lime-based renders is due to the natural processes both in the drying and carbonation process, but also in later natural physical and chemical processes in the render. Big patterns are often the result of a too fast drying and hardening process and internal stress results in these patterns. These cracks often occur a few hours after applying and should be pinched together while the render is still in its plastic phase. These plastic cracks should not be confused with the small-scale patterns (crazing) that develops due to the constant and natural exchange of CO2, moisture and lime. These patterns often don't appear until old paint has been removed. But in non-painted surfaces these patterns are easily recognised and visible. Non-treated they appear as porous and dusty surfaces with a low strength, but still with good adhesion to the substrate (no hollow sound).

Consolidation with water glass (soluble glass)

To preserve these original renders, a good alternative is to consolidate the surface with pure water glass (sodium silicate / soluble glass). Water glass reacts chemically with the lime-compounds in the render and forms a chemical reaction product that strengthens the render. It creates a porous crystal structure and still yields a high damp diffusion open system. Water glass is often also used as a dust binder on other mineral surfaces due to this effect. On weak and old lime surfaces we observe that this consolidation gives new life to the existing material. Be aware though that the reaction product may penetrate 6 mm deep and develop a higher strength reaction product than the original lime, and can cause layers to peel off. Always carry out a test. Consolidation with water glass can also be used on dusty mineral paints and as an extra impregnation in window sills. The water retention will be somewhat increased due to the changed crystal structure. The reaction product is purely natural and also used in stone conservation. The water glass technology is the basis of silicate paints.