Notes on toxicity

People who are new to natural plasters sometimes think they are non-toxic: earth plasters are made out of materials dug from the ground, right, how could they be dangerous? In fact, while the end result is non-toxic, these products can still be hazardous to work with. Take clay for instance, which often contains large amounts of crystalline, or ‘free’, silica (fine quartz), which when inhaled causes silicosis (a debilitating lung disease) and lung cancer. Silica is also found in cement and fine sand, but not in pure lime (which nevertheless isn’t great to breathe in). The long and the short of it is that plasterers work with materials in fine powder form and need to be very careful about what they breathe in.

  • Always wear a respirator when mixing, or anytime there is dust  – including cleanup!
  • Use a mop, or a vacuum with HEPA filter, instead of sweeping when fine plaster dust is present.
  •  Read the Material Safety Data Sheet (MSDS) for materials you are working with, including bagged clays and pigments

MSDS sheets can be found for most materials using an internet search. For example an internet search for “msds epk” finds that EPK bagged clay contains 0.1 – 4% crystalline silica, whereas the MSDS for Bell Dark Ball Clay shows it contains 10 – 30% silica. Kaolin clays often contain less than 1% silica, and are good for earth plaster finish coats. Ball clays and fire clays are more common in earth plaster base coats, and typically have large amounts of free silica. Site clays are typically processed wet, so they are hazardous only during cleanup.

Pigments vary greatly in their toxicity. Here’s a website that gives a summary of composition and toxicity of a few common pigments. The colour index can be helpful in trying to figure out what pigments are made up of.


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