Micro Organisms

Ceramic glazes and clay bodies can host micro organisms. They can be just a nuisance, a source of worry or can render a product useless. What should you do?

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Every potter has had the experience of opening a bucket of glaze only to be met with a foul odour; microorganisms of some sort have grown. Or, open a jar of brushing glaze only to find that it has thinned and does not brush on normally. While it seems odd that they could even have grown in a bucket of water mixed with rock powder, it is an aqueous system and usually, at least some organics are present (e.g. lignite in the clays, CMC gum). The microbials can lessen or neutralize the function of the organic binders in the slurry, thus affecting suspension, application properties and rheology. It follows that fixing the problem will not be as easy as just adding some bleach or Dettol. That may or may not remove the odor, but it won’t fix the damage done. Typically, a glaze like this is ‘terminal’. That is not to say there exists no strategy to correct it, but no one is going to hire a microbiologist to figure out whether it is a bacteria, fungus, mold, algae or yeast and what anti-microbial is needed! Glazes used in industry seldom have this issue and it is not because they are used continuously as made, it is because anti-microbials (biocides) are added when they are mixed. Just like with human health, little is needed to prevent the growth of microorganisms whereas a lot is needed to rescue a slurry once they take hold.

All prepared wet glazes made for pottery and hobby markets contain biocidal additives (the most likely target is bacteria). However, the prepared glaze market is small, so few or no additives are made explicitly for them. However, products like paint, enamel, building and agricultural products, and even cosmetics, are water suspensions and many proven biocides are available for them. But it can be difficult to find and choose one. Companies using these would generally send a sample of contaminated glaze for analysis and recommendation of an anti-bacteria or anti-fungal to prevent it. The percentages recommended are typically quoted based on the weight of the slurry (e.g. 0.25% of the slurry).

Getting biocides approved in a country can be a daunting task that only larger multinational companies can navigate, and the difference in regulatory environments means they may have to produce many variations of a product. This means that even if you find a product from DuPont, TroyCorp or Zschimmer that looks great, it may not be approved for use in your country. Thus, it is best to Google chemical distributors in your area to see what anti-microbials they import and distribute. Fortunately, if a biocide is good for aqueous slurries, paints, adhesives or latex, it should be good for glazes (in the proportion they recommend).

Ceramic supply stores for potters and hobby do not generally carry biocides. And issues with spoiled glazes are rare enough, unless of course you live in a hot damp climate, that DIY glaze making potters can live without them by simply avoiding bio-contamination when mixing and using the slurries, and avoiding storing them in overly warm places or in bright light. As a potter or hobbyist, what insurance against microbial could you employ? Simply adding few drops of Detol or bleach in newly mixed glazes might suffice. A possible biocide source is the DIY personal care products industry - they have preservatives graded from “wash off” to “leave in” products, including soaps, shampoos, moisturizers and lip balms. Percentage additions also range from 0.1%.

Is your contaminated glaze hazardous to handle? Likely not - but to be sure would be as difficult to establish as determining what biocide and how much is needed to fix it. The best course is to throw it out. That being said, it might be possible to dry it out to a powder, sterilize the powder and then remix it.

Micro-organisms do not just grow in glazes, over time mold can grow on pugged clay (stored in plastic bags). The amount and type of growth depend on temperature and available light, time and type of clay. The process is expected and natural. It might seem logical that porcelains, being made from higher purity materials and having a denser matrix and lacking larger particles, would grow less mold. But such is not the case. Often, the surface mold grows in small spots of intense color, dark enough that potters can mistake it for iron impurities. Patching patterns of discoloration (a slightly different shade of the clay itself) can penetrate into the clay from the surface. Inside the clay slug itself, aging can amplify the spiral pattern created by the pugmill, even creating zones of stiffer and softer clay, These are all manifestations of the aging process and seldom manifest an odour, it is thought these even improve the plastic properties. A few moments of wedging will typically bring the material back to its pristine production consistency.