Strips of the same glaze recipe, each containing different percentages of zircon opacifier, have been applied across both a dark-burning body and a white engobe. While it is difficult to measure the absolute degree of opacity from a photo like this, the side-by-side comparison makes differences easy to see. Tests like this demonstrate that simple visual comparisons can often be as useful as instrument measurements when evaluating glaze opacity. Colorimeters measure Lab* color values, not opacity directly. This test is really about visual hiding power, which instruments don't always capture well.

In this case the technician may be trying to determine how to achieve the required whiteness at minimum cost. Both the engobe and glaze contain zircon. The whiter the engobe the less opacifier is required in the glaze over it. Zircon is an expensive material, and reducing the total needed by even by 1% can make a significant difference in production cost.

The same test method can also be used to compare different brands of zircon opacifier in the same recipe.

A subtle aspect can be noted by coverage on the dark body: Opacity differences there become visible, whereas on the white engobe, differences almost disappear. Where an engobe is not to be used, an effective tweak is to add a thin black line under the glaze, then the lowest zircon level that hides the line becomes obvious.

Are your records in a messy binder? Binders don't have "Search" buttons. Side-by-sides. And many DIYers would generate a binder full in a year. But how does one even start to organize?

Start by moving your recipes to an account at insight-live.com, assigning each a code number. Then, in your studio/lab, label every fired sample, bucket, jar, glaze test, bag with the corresponding code number. Upload pictures for each recipe. Enter your firing schedules. Research the solutions to issues you are facing with glazes at the Digitalfire Reference Library (ask us questions using the contact form on each of the thousands of pages there). Then start planning improvements and tests. Choose a recipe you need to improve/evolve, duplicate it, increment the code number, make changes, enter explanatory notes. With this preparation you will hit the ground running back at work.

The mug on the left, #1, is a commercial brushing glaze. It is opaque enough to cover this red-burning clay body. It shows the desired effect. That depends on the fact that opaque glazes stretch thinner on the sharp edges of incised designs. If they have enough melt mobility and are applied right, the effect is amplified. This potter is attempting to mix her own DIY equivalent as a dipping glaze, adding 4% tin oxide to a transparent base glaze in #2 and zircon (a higher percentage) in #3. As you can see, the effect is not working as well, and there are several reasons:

#2 is whiter because it uses tin oxide as the opacifier (vs. zircon, likely used in #1). #2 and #3 have less melt fluidity; the base is likely G2926B. #1 was applied by brush, the others using a dipping glaze. Matching the original involves a combination of things: A base having more flux (especially B2O3) is needed (e.g. G3806C). Careful control of application thickness. The right percentage of opacifier. And, it may be necessary to mix the DIY version as a brushing glaze, that method of application might be needed to get the careful control of thickness and thickness variation needed.

Insight-live does not automate formula-to-batch calculations, but it does assist in doing them. And it provides the tools to create an audit trail of test results, pictures and notes and a path to document subsequent adjustments. Along the way, you gain material knowledge and intuition. In this example, we derive the recipe of materials needed to source the oxide formula of a zinc clear cone 6 glaze (sourcing the oxides needed using a Ferro frit and other common raw materials). We'll create the target in a panel, start the batch in a panel beside it, supply the B2O3 from a frit and then the fluxes from feldspar, zinc and whiting. Then finish by rounding out the Al2O3 and SiO2 from kaolin and silica. The picture below shows the panels, the original target formula on the left and the final derived recipe on the right. The derived transparent glaze is on the inside of the mug and the outside is G3875, another zinc clear with iron and chrome added to produce the orange.

This glaze is 85% Albany Slip and 20% Ferro Frit 3195. These bisque tiles were dipped in a brushing glaze version of it (just water and powder). The glaze is applied quite thin on the front tile and thicker on the back one. The material gelled slurries and required a lot of water to make them thin enough to use. For assured success, this or any glaze that had a high percentage required mixing the raw Albany Slip with a calcined Albany Slip (which people had to make on their own).

Oil-spot effects depend on being able to layer glazes. Normally, a black underneath and matte white over top. Using dipping glazes is obviously advantageous for this type of piece; the second dip covers the other half and creates the double layer needed. But dipping glazes contain clay in the recipe (almost always kaolin or ball clay), it satisfies multiple needs: It suspends the slurry, hardens the drying glaze, and supplies critical Al2O3 to the chemistry. The upper glaze here is a matte; so it needs extra Al2O3, which means it likely has extra clay. 20% kaolin (non-plastic like EPK, Grolleg, NZ) is about the maximum or the glaze will shrink too much when drying. If extra Gerstley Borate is added, then it will be worse. Drying cracks are the result when the fragile body-bond of the lower one fails to withstand the stresses of being rewetted and tugged upon by the upper layer being applied and drying. When the cracked double-layer begins to melt, it pulls itself into islands, leaving bare body between. That is called "crawling".

How to prevent this. Be smart about glaze layering and use recipe logic. The lower black can likely be adjusted to be a first coat dipping glaze. The clay content in the upper white one can be adjusted to reduce drying shrinkage (by splitting it between raw and roasted/calcined powder, or by using a clay having lower plasticity).

Small-scale operations everywhere are making tile like this. Most use plastic clay intended for pottery, which introduces more drying shrinkage, complicating drying them flat. Stacking them in the kiln can be a game of chance. Stacked too tightly and they crack (mostly because of quartz inversion). Stacked to loosely and most of the energy goes into heating the shelves and stackers. Using a clay with minimal large quartz particles is the best way to avoid dunting, however that is also a balance since such clays are more difficult to fit glazes to (without crazing).

Consider the hazards and hassles before choosing a black matte or gloss recipe that has high individual or combined percentages of manganese dioxide, cobalt or nickel.

Gloss blacks: These are super popular as the base for layering of reactive glazes. DIY dipping versions thus make a lot of sense. They make even more sense when they don’t turn to jelly in the bucket because of the high percentage of red iron oxide in all blacks made using metal oxide colorants. And when the total percentage of pigment is as high, or higher than 15%. And when the pigments cause crystallization (especially when overloaded).

Matte blacks: The human eye can detect even slight differences in the degree of matteness (which is very difficult to keep consistent). Raw metal oxides affect the matteness, especially when overloaded with pigment. They are prone to cutlery marking if too matte. By using stains, manufacturers and even potters have learned to tune recipes (lower left) and firing schedules to achieve consistency and functionality (even tourist souvenirs (lower right) feature them now). With stains, only one material is producing the color, its percentage (which can be as low as 4%) can be tuned.

This is the dolomite body recipe L4410P (a development version of Plainsman Snow). It is monoporosa tile on steroids; this body has zero percent firing shrinkage at cone 04! Predicting the final size and keeping that size consistent is much easier with such bodies. I have measured its drying shrinkage as 6% (doing our standard SHAB test). The final size needed is 20.5 cm square. Thus, I calculate the cut size to be 20.5 / (100 - 0.06) = 21.8 cm (or 20.5 / 0.94 = 21.8 cm). To keep these flat, we put them between sheets of drywall; the process takes 2-3 days. Since no change in size occurs during firing, this body has another big advantage: Tiles stay flatter during firing (a major problem with tile production). While making wall tile using a plastic pottery body is not something for industry (especially because of the space requirements for drying), for artisans working on a small scale, a body made by mixing super plastic ball clay with dolomite produces amazing working and tactile properties. The bonus is that they work so well at low temperatures, where there are so many glazing options.

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. Glazes that are very glassy but on the edge of structural instability do this. So they are not good for functional ware.

This is an adjustment to the 50:30:20 Gerstley Borate base recipe (historically used for reactive glazes, often on functional surfaces! This cuts B2O3 and adds significant SiO2. But it still has double the boron of a typical functional glaze. While the chemistry of the original was within the territory of boron blue development (relatively low Al2O3), this one is better because of the increased SiO2 (the high MgO:CaO ratio is likely also helping). Boron blues like the lower Fe2O3 content or Gillespie Borate. One more factor: I am using 325 mesh silica here, it dissolves in the melt better.