In some places and climates, this is more of a problem than in others. It is often not something that can just be ignored because the rheology of the slurry will likely be affected. Is there a magic chemical one can dump in to fix it? Not really. The subject of microorganisms in glaze slurries can be as complicated as you want to make it. This is because there are just so many different things that could be causing the stink. And there is no one chemical that treats them all. Even if there was, it's use would be focussed on prevention rather than fixing a problem. And, it would being its own issues, hazards and specific procedures. There are some simple things to know about dealing with microorganisms in glazes that should enable to you keep relatively free of this issue.

To make a low SG version of G2934BL I have already weighed out a 340g batch (it contains 5g each of Veegum and CMC gum to gel the slurry and slow the drying). I use 440g of water initially (adjusting that according to experience in brushing behaviour). After shake-mixing all the powder in the plastic bag I pour it into the water on low speed and finish with 20 seconds on high speed. This produces a low specific gravity brushing glaze, it just fills this 500ml jar. In subsequent batches, I adjust the Veegum for more or less gel and the CMC for slower or faster drying. Later I also assess whether the CMC gum is being degraded by microbial attack - often evident if the slurry thins and loses its gel. Since each glaze recipe responds differently and changes differently over time, good notes are essential. We are working on dozens of these at any given time, each is code-numbered in our group account at Insight-live.com. This is so worthwhile doing that I make quality custom labels for each jar!

The clays are Plainsman H450 and H550. Firing is cone 10 reduction. A 50:50 mix of roasted and raw Ravenscrag slip was used. L3954N black engobe was applied at leather hard stage (on the insides and partway down the outsides). We call this recipe GR10-C Ravenscrag Talc Matte, it is on the insides of both and on the outside of the one on the left. The outside of the other is G2571A Bamboo, it is also an excellent matte base. The silky matte surfaces produced by these two are both functional (they are very durable and do not stain or cutlery mark). And they are very pleasant to the touch.

The L4655 floating blue recipe is on the outside of the mug. It adds titanium to the GA6-A base. We wanted to reduce the thermal expansion to minimize the likelihood of crazing. So the obvious question was: Could we substitute the Ferro Frit 3134 for Frit 3195 in the base (effectively using GA6-B instead of GA6-A)? The calculation showed that the thermal expansion should drop from 7.6 to 7.2. Unfortunately, it did not work. The two tiles in the front show that (the one on the right adds 2% iron, we thought that might enhance the rutile blue effect). Why did this fail? Likely the raising of the Al2O3 makes the melt stiffer, that is preventing the freedom of movement needed to form the crystalline phases.

Almost all ceramic glazes are a base recipe with additions of colors, opacifiers, variegators, etc. Our traditional G3933 oatmeal glaze is a good example (recipe on the left). It can produce rich brown silky matte surfaces, especially on dark burning bodies. But one problem has emerged: The tendency to crawl. Much testing has yet to reveal the reason. Would it be possible to base the recipe on Ravenscrag Slip and achieve the same chemistry? Yes. And some unexpected benefits accrued. In the recipe on the right I sourced MgO (the key to the matte surface) from dolomite and Ferro Frit frit 3249 (earlier tests sourcing from talc were unsuccessful, off-gassing from the talc was puffing up the glaze with micro-bubbles). The all-new G3933E recipe has the same chemistry (I derived it in my account at insight-live.com). It is not likely to be without problems, but it looks identical (with richer color from a little more iron oxide), it does not crawl and it's recipe and chemistry are flexible. It is glossy when cooled fast and silky matte when cooled slowly. The MgO can be increased easily to get matteness with quick cool also. The mix of calcine and raw Ravenscrag Slip also enable control over the slurry and application properties.

The melt fluidity tester was fired at cone 6. The glaze on the left is G2826A2, a 50:30:20 Gerstley Borate glaze historically used for reactive glazes. The one on the right is G2926A3, an adjusted version that cuts the B2O3 level and adds lots of SiO2. The result is much more sane, although still very melt-fluid glaze. This is also a lesson in the chemistry that produces boron-blue, the one on the left does not and the one on the right does. This is the most decorative boron-blue we have ever seen, especially on dark bodies. Why? High B2O3 is not the key, it is lower. CaO is lower but it was higher in the original 50:30:20 recipe and that had plenty of boron blue. The SiO2 appears to be the enabler, it is much higher. And we are using 325 mesh silica, so it dissolves in the melt better.

This is G2826A3, a transparent amber glaze at cone 6 on white (Plainsman M370), black (Plainsman 3B + 6% Mason 6666 black stain) and red (Plainsman M390) stoneware bodies. When the glaze is thinly applied it is transparent. But at a tipping-point-thickness it generates boron-blue that transforms it into a milky white.

This is a variation on the G2926B cone 6 pottery glaze recipe, it contains 22% Gillespie Borate (GB) and 12% calcined kaolin. Our objective was to create a glaze of the same chemistry but source the B2O3 from GB instead of a frit. While the fired result was near perfect (exactly matching the melt fluidity and crystal clear) the crawling is so bad that it is almost usable. The reason was not obvious until we fired a sample to 1550F and 1650F. At the former the integrity of the glaze layer is great, but by 1650F it does this (many of the edges of these are curling upward). Ulexite, which GB contains, is known for the behaviour of suddenly shrinking and then suddenly melting over a narrow range of temperatures. Since GB is plastic and suspends slurries well we thought calcined kaolin would be better than raw kaolin (to minimize drying shrinkage). However, it is making things worse (with raw kaolin the crawling was much less).

Potters often store glazes for long periods so tiny spherical precipitate particles can form. These were found in a months-old bucket (about 2 gallons). These can appear over time, depending on factors like temperature, electrolytes in your water or solubility in the materials (likely, the frit is slightly soluble). The glaze slurry should be screened periodically (or immediately if you note the particles when glazing a piece). This is an 80 mesh sieve. Note the brush, using one of these gets the glaze through the screen much quicker than using a rubber spatula. The loss of material on the screen is tiny and inconsequential to the glaze. But it is crucial because these particles do not melt at cone 6, they will certainly mar the fired glaze surface if undetected.

Left: A small dry lump has been immersed in water (top is a sodium bentonite from Saskatchewan, bottom is a calcium bentonite from Mexico). Right: After ten minutes both have swelled. Although the character of the swelling is a little different, both dewater on a plaster table and exhibit similar plasticity. Both create a gelled slurry having similar feel and character. This seems odd. The lesson appears to be that all sodium bentonites are not created equal. Sodium bentonites normally resist wetting, they literally cannot be stirred into water without the use of high-energy mixing equipment. And tiny amounts can gel a large amount of water or plasticize an otherwise short clay body. However, that is not the case with this sodium bentonite we are getting from Saskatchewan. This is exciting for us because it suggests that this material, although not plastic enough for use in ceramics, can be used for cosmetics, masks and detox.