It is about the recipe and the firing temperature. This is Plainsman BGP, a terra cotta, with 30% dolomite. Note the "DSHR" column in the SHAB test data (third last column): The drying shrinkage still averages over 7% even with the 30% dolomite, so BGP is very plastic. Notice the "FSHR" column, it is negative for the first five test bars fired at cone 05-01, that means the bars grew in size! But notice the shrinkage hits 0% at cone 1 (bar #6), by cone 2 the trend has reversed to 0.3% shrinkage. Amazingly that #6 bar is approaching vitrification, it is strong. This body was intended as a high-porosity ceramic at the lower ranges, at cone 04 is so porous that it can be used as a filter.

These are actually two different cone 6 base glazes to which I add a black stain. I trust them because I formulated and perfected them myself: G2926B clear and GA6-B Alberta Slip base. They are durable, fit my clay bodies, melt well yet can host a stain without loss of gloss. I even know the chemistry, both have plenty of SiO2 and Al2O3, that is a hallmark of durability. I fired these using the PLC6DS schedule. I add 5% black stain to the former and 4% to the latter, both yield a jet-black. The GA6-B requires ball milling. Stains are inherently much safer to use than raw metal oxide colorants because they are sintered as colorant/stabilizer blends. And much less is needed. Contrast that with raw metal oxides, it is common to find black recipes containing up to 15% of blends of nickel, cobalt, iron and manganese! At times the manganese alone can be 8% or more! So, I feel relatively safe using these coloured glazes on a surface that will be exposed to hot and acidic liquids.

This clay, L4115J3S, a Plainsman 3D-based experimental body, fires vitreous and dense (it contains 0.2% granular manganese). These glazes are very durable and functional. The outside glaze on both is G2934W (adds 10% zircopax). In our C6DHSC firings this produces as matte a surface as is possible without having excessive staining problems. Left mug inside glaze: An 85:15 mix of G2934 matte (without zircopax) and G2926B clear glossy. Right mug inside: G2926B clear glossy ball-milled, over this body it produces a striking visual surface. These mugs look as close to cone 10R dolomite-glazed ware as we have ever seen!

Why do this? We did not have it in stock and customers needed to mix recipes. When the chemistries of the two feldspars are very similar substitution is often not a problem, especially when a recipe only calls for 5 or 10%. However, when a recipe calls for a significant percentage the situation becomes much trickier (in our cone 6 test recipe, "Perfect Clear", 40% Minspar is needed). Feldspars are almost a glaze in themselves, just needing silica and alumina to shift their chemistry toward 'glazedom'. In this project I calculated a mix of materials, in my Insight-live.com account, that sources the same chemistry as Minspar. I made a cone 6 GLFL test comparing the Minspar and Minspar substitute (left) and comparing the Perfect Clear glaze with each feldspar (right). As you can see, the similarity in melt flow is stunning! This is a real demonstration of just how practical and valuable glaze chemistry calculation can be.

Before jumping to conclusions consider all the factors that relate. This is M340S, it is fired at cone 6. That temperature is a "sweet spot" for this effect, high enough for the particles to bleed and low enough they do not bloat the body. Such bodies contain only about 0.2% of 60-80 mesh granular manganese (compare this to many glazes that employ 5% powdered manganese as a colorant). Further, the vast majority of the manganese particles are encapsulated within the clay matrix. The tiny percentage exposed at the body surface are under the glaze. It is not the manganese particles themselves that expose at the glaze surface. Rather particle surfaces that contact the underside of the glaze bleed out into it from below, doing so as a function the glaze thickness and melt fluidity. Thus, food contact with a glass surface having isolated manganese-pigmented regions is not at all the same thing as with raw manganese metal. Consider also that the total area of manganese-stained glass on a functional surface is extremely small for this effect.

At low temperatures glazes and slips/engobes are not stuck on nearly as well as with stoneware and porcelain. So the glaze fit has to be better (poor fit will be evidenced by flaking at the lip). But that is not what is happening here. In this case a pigmented slip, or underglaze, was applied first, at leather-hard stage (thus it is being used as an engobe). The integrity of two bonds must now be considered: Slip-with-body and glaze-with-slip. Slip-to-body bonding is never as good as glaze-to-body or glaze-to-slip. When an engobe, or underglaze, is refractory then the bond-with-body is especially poor. Ceramic stains are highly refractory in comparison with low-fire bodies, simply adding them to an underglaze base recipe will make it refractory also. In addition, stains vary widely in their refractory character and the percentage of stain needed varies greatly with color. Some underglaze manufacturers compensate by incorporating a compensatory percentage of frit in each underglaze recipe. Other manufacturers simply have one base and add all the colours to that. Claims that underglazes work well across wide temperature ranges do not get tested when they are brushed on as decoration, but when they are applied like this, as an engobe, disaster strikes! In this case we can see that the failure is occurring at the underglaze-body interface and the glaze/underglaze "sandwich" is releasing in large flakes.

I was impressed with this frit company, Reimbold & Strick. Their frit data was educational, by studying the chemistry of their matte frits, for example, you could see the various mechanisms that produce the effect. But, it is no longer available. However, with a little determination you could likely find the pages at the internet archive (see link below).

On paper, Fusion F-19 has a very similar chemistry to 3124. However, as can be seen here, it is flowing a little more and appears to have a lower surface tension. The glass is also more transparent and the entrained bubbles are bigger. The differences could be partly to Fusion using a different set of raw materials to source the chemistry or differences in their smelting process.

On paper, Fusion F-12 has a very similar chemistry to 3134. And in practice it also appears very similar, although a little more melt fluid than the 3134.

The Fusion one is flowing a little more. And it has larger bubbles, so Fusion must be using a different set of raw materials to source the chemistry (materials that either have higher LOI or decompose to produce gases later in the firing). They do not supply the chemistry of Frit F621/19 and it is not shown on their website in 2021. But they recommended it to us as substitute candidate for Ferro Frit 3124.