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Where Do I Start?

Section: Glazes, Subsection: Introduction

Description

The perfect universal glaze recipe does not exist, the only way you will get the glazes you really need is formulate or adapt them yourself. Start with base recipes, learn to understand them from a material level, then learn the mechanisms, and chemistry.

Article Text

Probably you are reading this because you are starting to mix your own ceramic glazes and are overwhelmed. Or you have done something a certain way for years and now it is not working. Or your company expects you to fix problems for which you do not have the training.

There is a prevailing culture in pottery, hobby ceramics and even industry of getting by with the least knowlege possible. Somehow we want to believe that we can mix a recipe we find on Pinterest.com and it will magically work just like the picture. You may have realized by now that this site is dedicated to fighting that culture (which we personify as "the glaze dragon"). We recommend that you embark upon a journey of understanding as follows:

Are you at a school, art center or club?

Are you at a factory?

Places to start

If you are going to make ceramic ware, put good glazes on it. Remember, a glaze is a lot more than one that just has a pleasing fired appearance. There is no one-glaze-that-works-for-everyone. We cater to people that want to start out right, or have been kicked around long enough that they are ready to learn why, they want to "understand". You will never likely get the glazes you really want until you formulate or adapt them yourself.

The traffic in glaze recipe will burn your success!

A good matte glaze. A bad matte glaze.

A good matte glaze. A bad matte glaze.

A melt fluidity comparison between two cone 6 matte glazes. G2934 is an MgO saturated boron fluxed glaze that melts to the right degree, forms a good glass, has a low thermal expansion, resists leaching and does not cutlery mark. G2000 is a much-trafficked cone 6 recipe, it is fluxed by zinc to produce a surface mesh of micro-crystals that not only mattes but also opacifies the glaze. But it forms a poor glass, runs too much, cutlery marks badly, stains easily, crazes and is likely not food safe! The G2934 recipe is google-searchable and a good demonstration of how the high-MgO matte mechanism (from talc) creates a silky surface at cone 6 oxidation the same as it does at cone 10 reduction (from dolomite). However it does need a tin or zircon addition to be white.

Do you know the purpose of these common Ferro frits?

Do you know the purpose of these common Ferro frits?

I used a binder to form 10 gram balls and fired them at cone 08 (1700F). Frits melt really well, they do not gas and they have chemistries we cannot get from raw materials (similar ones to these are sold by other manufacturers). These contain boron (B2O3), it is magic, a low expansion super-melter. Frit 3124 (glossy) and 3195 (silky matte) are balanced-chemistry bases (just add 10-15% kaolin for a cone 04 glaze, or more silica+kaolin to go higher). Consider Frit 3110 a man-made low-Al2O3 super feldspar. Its high-sodium makes it high thermal expansion. It works in bodies and is great to incorporate into glazes that shiver. The high-MgO Frit 3249 has a very-low expansion, it is great for crazing glazes. Frit 3134 is similar to 3124 but without Al2O3. Use it where the glaze does not need more Al2O3 (e.g. it already has enough clay). It is no accident that these are used by potters in North America, they complement each other well. The Gerstley Borate is a natural source of boron (with issues frits do not have).

Do your functional glazes do this? Fix them. Now.

Do your functional glazes do this? Fix them. Now.

These cone 6 porcelain mugs have glossy liner glazes and matte outers: VC71 (left) crazes, G2934 does not (it is highlighted using a felt marker and solvent). Crazing, while appropriate on non-functional ware, is unsanitary and severely weakens the ware (up to 300%). If your ware develops this your customers will bring it back for replacement. What will you do? The thermal expansion of VC71 is alot higher. It is a product of the chemistry (in this case, high sodium and low alumina). No change in firing will fix this, the body and glaze are not expansion compatible. Period. The fix: Change bodies and start all over. Use another glaze. Or, adjust this recipe to reduce its thermal expansion.

In pursuit of a reactive cone 6 base that I can live with

In pursuit of a reactive cone 6 base that I can live with

These melt-flow and ball-melt tests compare 6 unconventionally fluxed glazes with a traditional cone 6 moderately boron fluxed (+soda/calcia/magnesia) base (far left Plainsman G2926B). The objective is to achieve higher melt fluidity for a more brilliant surface and for more reactive response with colorant and variegator additions (with awareness of downsides of this). Classified by most active fluxes they are: G3814 - Moderate zinc, no boron G2938 - High-soda+lithia+strontium G3808 - High boron+soda (Gerstley Borate based) G3808A - 3808 chemistry sourced from frits G3813 - Boron+zinc+lithia G3806B - Soda+zinc+strontium+boron (mixed oxide effect) This series of tests was done to choose a recipe, that while more fluid, will have a minimum of the problems associated with such (e.g. crazing, blistering, excessive running, susceptibility to leaching). As a final step the recipe will be adjusted as needed. We eventually chose G3806B and further modified it to reduce the thermal expansion.

Out Bound Links

In Bound Links


By Tony Hansen




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