Clay Craze

I received a call recently from a contractor who had applied American Clay three times on a project and it kept on cracking. It sounded a lot worse than it actually was. These areas were more smaller "crazes" than buckling "cracks", but still it needed to be resolved for a happy client.

Binders such as clay and lime once applied as a plaster lose all of their water to evaporation. If there is excessive water, the plaster is not compressed whilst drying small to medium cracking can occur called ‘crazing’ or ‘checking". Clay contains more water content than lime plasters. It also requires you to add water back to the surface while compressing it, so extra caution should be paid on this critical final step. Compressing ensures that the layers of applied clay have been pushed together to form a mechanical bond to the substrate. It's critical for a smooth and solid finish.

 In addition to the cracks, there was also greyish burn marks on the surface from using a steel trowel. The friction from using your trowel to compress the wall causes the plaster to turn grey from the steel material. Metal is heat conductive so a plastic trowel should be used especially on lighter surfaces. Both the burnish marks and the behavior of the cracks indicated it was a an issue in compressing the clay rather than a product or substrate failure.


Here's my prescription for bringing the beauty back!

• Scrape down any loose plaster and apply a sanded primer to the surface. This might be overkill, but creates a new vapor barrier and layer for the old and new plaster to adhere to.

• Use a plastic trowel to compress lighter colored surfaces to avoid unsightly grey burn marks.

• Compress clay when it goes to "set", "tack-free", or "leather-hard". This is when you can press your finger into it and no plaster is removed. You also know it's the right time to compress when the color starts to lighten. This requires less water to be used, which minimizes potential for cracks to form.



Clay - Ready for compressing!