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Limit Recipe

Because of the proliferation of glaze recipes online, potters and technicians need to develop critical thinking skills to be able identify obvious issues with new recipes presented to them. And testing skills to learn the less obvious ones. And material knowledge to recognize the mechanism of the fired appearance. And chemistry skills to evaluate that mechanism on the oxide level. And skills to fix problems, substitute materials, adjust surface/temperature or just transplant the mechanism into a base you already have working in your circumstances. The knowledge needed is here on this site, and its exciting and easier to learn than you might think.

The concept of a "limit recipe" is that we expect the percentages of material types to fall within certain ranges for typical glossy and matte functional or service glazes. If they are not red flags should pop up. Here are some examples:

Glazes need clay to suspend the slurry (and supply Al2O3 to the chemistry). We expect 10-30% in most. If a glaze has 5% clay, something is wrong. Perhaps it has 40%, that might be required to supply the chemistry needed for a matte but it will shrink and crack on drying.

SiO2 is the building block of all glazes. Most combinations of materials do not supply enough so most recipes contain silica. 10-30% is normal. What is there is none? Or 40%. Why?

Middle and low temperature glazes need significant boron (B2O3) to melt them. It comes from Gerstley borate and frits. What if a glaze has not of these? Perhaps it employs zinc or lithium as melters. But these are troublesome, are you ready for that?

Does it contain a feldspar you don't have? Is the chemistry available or is it at least know if it is a potash for soda feldspar that you could substitute. Does it contain a kaolin and ball clay that you do not have? Likely you can substitute one you do have. But being willing to do the testing is important.

Does it contain materials you have never heard of? Walk away. Or Cornwall Stone or Nepheline Syenite, these are similar to feldspar, you’ll need to calculate. Does it specify a substitute of these?

It is normal to see 1% cobalt, it is a powerful blue color. And super expensive. If there is 5% that is crazy. 3% copper oxide is normal, 20% is insane in a functional glaze. Carbonate colors (like copper, cobalt) are trouble, they gas during firing and produce blisters, the oxide forms are better. Stains are better yet.

Barium carbonate at 5% might be OK, but 20% is going to produce a toxic glaze. Lithium carbonate likewise.

High percentages of calcium carbonate or dolomite? They gas like mad, better to source CaO and MgO from wollastonite and talc.

Are you going to do multi-layering? Then you need to know how to turn the recipe you find into a first-coat dipping or brushing glaze.

Don't waste time. It is crazy to have a dozen completely different recipes in your operation. Learn from what commercial glaze manufacturers do. They create a base transparent and add colors, opacifiers and variegators to it to create an entire line of glazes. They use frits and stains, they are so much less troublesome and safer than raw fluxing materials (like lithium carbonate, calcium carbonate, zinc oxide, strontium carbonate, barium carbonate) and raw metallic carbonates and oxides (like copper, cobalt, chrome, iron). Don't try to save money, you will end up wasting time and reject ware.

Predict glazes likely to craze, leach, run, settle, crystallize, pinhole, blister.

Under construction.

The traffic in glaze recipes will burn your success!

The traffic in glaze recipes will burn your success!

They might look great on a fancy website, but what are the chances they will actually work in your circumstances? Very low. After trying many glazes you may think you have found one that works. But does it really? Or is it erratic and unreliable? Difficult to use. Does it leach or craze or shiver or pinhole or blister? Or give you other problems? Be critical and cautious about recipes you find.

Trafficked online recipes waiting for a victim to try them!

Trafficked online recipes waiting for a victim to try them!

Last week a customer came to buy materials to mix these recipes she found online. Then we had a closer look. Many have 50+% feldspar/Cornwall/nepheline with little dolomite or talc to counteract their high thermal expansion, these are guaranteed to craze. Many are high in Gerstley Borate, it will turn the slurry into a bucket of jelly. Some have high tin, lithium and cobalt; exceptionally expensive materials. Many have metal carbonates, which can produce blisters and bubbles in the fired surface. Some contain almost no clay, they will settle like a rock in the bucket. A better way? Identify the mechanisms (colorants, opacifiers and variegators) in each and transplant these into your own base transparent glossy and matte recipes. These already fit your bodies, have good slurry properties and are stable. Use stains instead of metal oxides when possible.

We fight the dragon that others do not even see

We fight the dragon that others do not even see

There are thousands of ceramic glaze recipes floating around the internet. People dream of finding that perfect one, but they often only think about the visual appearance, not of the usability, function, safety, cost or materials. That resistance to understanding your materials and glazes and learning to take control is what we personify as the dragon. Using the resources on this site you could be fixing, adjusting, testing, formulating your own glaze recipes. Start with your own account at insight-live.com.

An example of how much Gerstley Borate LOI can affect a glaze

An example of how much Gerstley Borate LOI can affect a glaze

Fired at cone 6. The samples on the bottom tiles are from ten-gram balls that have melted down. These glazes have the same chemistry, but the one of the left sources its B2O3 from Gerstley Borate (which has a high LOI). The one on the right gets it from a frit. Because the fritted version has less gases of decomposition to expel, the glass is much smoother. Curiously, the fritted version is flowing less and the red color has been lost. Why? This could be because the Al2O3, which stabilizes glazes against excessive fluidity, is being dissolved into the melt better and more available for glass building.

Out Bound Links

  • (Glossary) Target Formula, Limit Formula

    The term 'limit formula' historically has typically referred to efforts to establish absolute ranges for mixtures of oxides that melt well at an intended temperature and are not in sufficient excess to cause defects. These formulas typically show ranges for each oxide commonly used in a specific gla...

  • (Glossary) Mechanism

    Identifying a mechanism means you identify the reason a glaze does something specific. Especially visual. Most glaze recipes can be separated into two parts: the base and the mechanism of the color, opacity and variegation. The base is likely just a transparent glossy or translucent matte (although ...

  • (Glossary) Food Safe

    In recent years potters and small manufacturers have become aware (or have been forced to become aware) that ceramics and pottery are not as inert as they once thought. A variety of potential health impacts exist to users of ware they make. These include flaking off of glaze chips (that could be ing...

  • (Glossary) Frit

    A ceramic glass that has been premixed from raw powdered minerals and then melted, cooled by quenching in water, and ground into a fine powder (search youtube for interesting videos). Huge quantities and varieties of frits are manufactured for the ceramic industry every year (especially for tile) by...

  • (Glossary) LOI

    Simplistically, LOI is the percentage of weight a material loses on firing. Assuming firing to a typical stoneware temperature of 1200C, the amount of weight loss can be surprising. Kaolins, for example, lose around 12% (mainly crystal-bound water). Ball clays lose about half of that (a combination ...

  • (Glossary) Stain

    Stains are man-made colored powders used in glazes, bodies and engobes. They are manufactured by sintering powdered components in special furnaces at high temperatures. The powdered mixtures are stoichiometric, carefully compounded, finely ground and vigorously blended so that adjacent particles rea...

  • (Glossary) Brushing Glazes

    The ceramic hobby casting industry has long used commercially prepared, gummed, paint-on glazes. In recent years the hobby pottery community has followed suit. Even professional potters, who in the past made their own glazes, have embraced the use of pint and gallon bottles of brushing glazes for so...

  • (Glossary) Dipping Glazes

    In ceramics and pottery dipping glazes can be of two main types: For single layer or as a base for the application of other layers overtop. Dipping is the preferred method of application for single layer inside functional surfaces. Drying is quick and coverage is even if the slurry has the right...

In Bound Links

  • (Glossary) Glaze Recipes

    We have included this page to warn you about recipes that you find online. Be careful about wasting your time. And money. Do not think you will find a magic solution to all your dreams! What you will more likely find is alot of frustration and a stock of useless materials. Better to have a few recip...

By Tony Hansen

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