Dipping glaze applies and dries in seconds. Brushing glazes dry slowly and dry hard. Brushing glazes simply have gum in the recipe, dipping glazes typically do not. This is the Alberta Slip cone 6 base, made jet black with 4% black ceramic stain (our code G3914A). We normally mix this as a dipping glaze but I have made a 500ml jar of low SG brushing version using both of the following methods. These methods will work for almost any glaze recipe (for those having exceptionally high clay content less Veegum is needed).

1) Shake together, in a plastic bag, 340g of mixed glaze powder with 5g of Veegum and 5g of CMC gum. Add that to 440g of water in a kitchen blender and mix on high speed until it gels (the gums resist mixing so the highest speed for at least 30 seconds is needed to prevent lumps).

2) Take 680g of dipping glaze (assuming it is about 50:50 water:powder you get 340g of dry), put that in the blender with another 80g of water and proceed as in method 1. Less total water is being used because the dipping glaze might not be exactly 50:50 water:powder. During mixing, if it gels too much add the extra 20g of water.

This is so exciting that I make fancy labels, they are just ink-jetted onto regular paper, cut 62mm wide (2 7/16") and held securely on with 2 7/8" transparent packing tape.

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 making of Tandoor ovens seems ordinary to someone from a country where they are made. But it seems impossible to me (based on my lifetime experience working with clay). I would love to learn from someone in India the answers to these questions (based on the video links below), can you please message me if you can help?

-Clay shrinks when it dries. The more water it has the more it shrinks. When it dries unevenly it shrinks unevenly. Uneven drying means it cracks. The rim on these Tandoors should dry and become rigid long before the bases. Cracks should thus occur part way down the wall as the base tries to shrink against the already-dry rim. Why is this not happening?

-Dried clay is not rock hard, it can be turned back into mud if it gets wet. Why don't those ovens disintegrate in the rain?

-To do what the craftsmen and women do with the clay it would need to be very plastic (and thus have high drying shrinkage). Where the crew is building the six ovens beside the carriage the friction and drag on the ground should cause them to form cracks from the ground upward. Or they should pull to an oval shape. Why does this not happen?

-Wet clay cannot be joined to dry clay. The wet clay needs to shrink. How is it possible they are doing exactly this?

-Am I underestimating the degree to which the straw addition reduces the shrinkage of the clay? But plastic clay needs to shrink 10% whether there is straw or not. How can this work? Even if the straw could cut shrinkage to 5% that is still 1.5 inches on a 30 in diameter Base.

-Are they using a body high in ball clay? How else could the clay be plastic enough for this work if not?

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 50:30:20 cone 6 pottery glaze recipe, it contains 22% Gillespie Borate (GB) and 12% calcined kaolin. Our objective was to reduce its melt fluidity. But the crawling is so bad that it is almost unusable. 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, the improvement is minimal.