Lead bisilicate with his ugly borosilicate cousins at a cone 05 party

The middle front mug is glazed with an 85:15 lead bisilicate:kaolin mix, the G3971 recipe. It is an absolutely "knock your socks off" crystal-clear hyper-glossy surface that transmits the terra cotta color beautifully regardless of whether the clay is smooth or coarse or the glaze thick or thin (this one was applied as a brushing glaze in three coats on L215). My lead testing kit passes it with no detectable lead release. The other pieces are done using brush-on versions of boron-based clear glazes (commercial and made from a recipe). At almost any thickness and whether on L215 or L4170B clouding occurs. The worst one is a commercial three-coater on the right, the best is G1916W (it has 2% added iron as a fining agent for the micro-bubbles). My terra cotta plan: Glaze the inside functional surfaces with that and the outsides with the leaded one (and using a kiln exhaust system).

Related Pictures

Boron blue in low fire transparent glazes

This high boron cone 04 glaze is generating calcium-borate crystals during cool down (called boron-blue). This is a common problem and a reason to control the boron levels in transparent glazes; use just enough to melt it well. If more melt fluidity is needed, decrease the percentage of CaO. There is a positive: For opaque glazes, this effect can actually enable the use of less opacifier.

2% iron oxide in a glossy terra cotta glaze gives better color, less clouding

Two brilliantly transparent glazed terra cotta mugs

Both pieces are the same clay body, Plansman L215. Both are fired to cone 03. Both are glazed using G1916Q borosilicate recipe. The glaze on the piece on the left has 2% added iron oxide (sieved to 80 mesh). Each particle or agglomerate of iron (which is refractory in this situation) acts to congregate the micro-bubbles so they can better exit the glaze layer. Notice also how much richer the color is as a result. The piece on the right, without the added iron oxide, is neither as red nor as transparent. Of course, I had to be careful not to apply the glaze too thickly on both.

A lead bisilicate frit fails a leach test. Yet as-a-glaze it passes. Why?

I have soaked a lead bisilicate frit in vinegar overnight. To test whether it is leaching I pour the vinegar leachate into a test tube, soak a Q-Tip in the sensor solution and dip it into the vinegar. It turns black immediately - so we have lead in the leachate! But this is not as it seems.

Remember a key point here: The frit glass had no opportunity to be annealed - it was crash-cooled by being quenched in water. Annealing and associated toughening of the surface is a natural consequence of a glaze cooling slowly in a periodic kiln - which is why pieces made using an 85:15 mix of this same frit and kaolin, pass this test. The same 85:15 mix also still passes a lead check test if melted into an ingot and crushed into a granular powder (this is amazing given the exponential increase in surface area).

Is it impossible to get a good clear over-glaze for cone 6 brush decorated stoneware?

The mug on the right is terra cotta slipware firing at cone 04 using underglazes and a leaded transparent over-glaze (lead glazes are still commonly used in many parts of the world and considered safe). Mug on the left: This potter wants to use the same technique on cone 6 stoneware. Like pretty well all cone 6 recipes, this transparent depends on a boron frit to melt it. The result is micro-bubble clouding and boron blue when only slightly too thick. Also surface defects, washing out of colors and bleeding of brushwork. Actually, these commercial underglazes are an issue, by cone six they melt and diffuse into the clear overglaze. So is it possible to glaze-fire cone 6 stoneware and achieve results like the one on the right? At this time I do not know how! The path could be stained non-melting underglazes, separate body and glaze firings (the latter being much lower) and a transparent brushing glaze whose thickness and laydown can be carefully controlled.

Thick application clouds a transparent glaze on a terra cotta clay

Glaze clouding is a universal issue in ceramics. Terra cotta bodies demonstrate this best. Pretty well all transparent glazes, even commercially available ones, can cloud. This example is G2931K, it can be beautifully crystal clear. But the thickness of application is the key to achieving that (as thickness increases this happens). We ball milled it to see if that would help, but as you can see, that has not impacted the problem. This is a dipping version so that is part of the reason why it is easy to get it on too thick. One of the advantages of brushing glazes is the ability to carefully control thickness,
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A cure for long-time Gerstley Borate sufferers

Glazes using frits instead of Gerstley Borate

These are various different terra cotta clays fired to cone 04 with a recipe I developed that sources the same chemistry as the popular G2931 Worthington clear (50:30:20 GB:Kaolin:Silica) but from a different set of materials. The key change was that instead of getting the B₂O₃ from Gerstley Borate I sourced it first from Ulexite (G2931B) and then from a mix of frits (G2931K). All pieces were fired with a drop-and-hold firing schedule C03DRH. Fit was good on many terra cottas I tried (pieces even surviving boiling:icewater stressing). Where it did not fit I had thermal expansion adjustability because more than one frit was sourcing the boron. Frits are so much better for sourcing B₂O₃ than Gerstley Borate (the latter is notorious for turning glaze slurries into jelly!). Of course, a little glaze chemistry is needed to figure out how to convert a recipe from Gerstley Borate bondage to frit freedom, but there is lots of information here on how to do that.

Links

Glossary	Lead in Ceramic Glazes Lead is a melter in ceramic glazes and performs exceptionally well and must be misused to be toxic. It is also now environmentally pervasive. It is toxic and cumulative at any level of exposure.
Materials	Lead Bisilicate Frit A standard frit of 1 molar part of PbO and 2 of SiO2. It is considered stable and non-leachable.

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