All Glossary

Low Temperature Glaze

In ceramics, glazes are loosely classified as low, medium and high temperature. Low temperature is in the cone 06-2 range (about 1800F-2000F).

Key phrases linking here: low-temperature glazes, low temperature glaze, low-temperature glaze, cone 04, cone 06 - Learn more

Details

This term generally refers to glazes that mature from cone 06-04. They are obviously not as functional as stoneware and porcelain (less durable) and are generally targeted at decorative ware. Orton cones in the 04-02 range span much wider temperature regions (measured in degrees) than those at higher stoneware and porcelain ranges, so working range is actually quite wide and flexible. For example, glazes that work at cone 06 also often work as well cone 02, almost 200F higher!

Historically, lead compounds were used to melt glazes at low temperatures, but they have fallen out of use due to toxicity issues. Now the melter is boron, it is almost universally supplied by frits (historically, Gerstley Borate and Colemanite were used by potters to source boron). The entire prepared-glaze industry is built on hundreds of general-purpose and specialized boron frits. The existence of these, and ceramic stains and opacifiers, enables companies (and individuals) to create amazing glazes.

While low-fire glazes are not as hard as well-formulated medium-temperature glazes, they have the advantage of supporting bright colors and very high gloss. For end-users, generally, the major challenges are to find a body that fits the glazes they want to use (without crazing or shivering), achieve even coverage and be able to fire ware with a minimum of surface defects. Low-temperature white bodies fire porous out of necessity (there is no practical way to make them vitrify). They thus have relatively low strength. But they are tolerated because they provide a base that brings out the color. Terra cotta bodies provide extra fired strength (but often need an underglaze or engobe to enable bright colored glazes).

Colorless glazes do not need to contain toxic materials so they can be used on functional surfaces without issue (although the surface gloss can degrade over time). Manufacturers claim that their brightly colored glazes are food safe if used according to directions. That being said, there is need for caution, especially among hobbyists. There is no getting around it, bright red glazes contain cadmium stains.

At stoneware temperatures, the fired properties of glazes can usually be predicted from their chemistries. But at low temperature this is much less the case. The oxides commonly regarded as fluxes at middle and high fire are often simply fillers (and even matting agents) at low temperatures; thus understanding the relationship between chemistry and fired physical properties is more challenging.

The hobbyist paint-your-ware market is mostly outside of the scope of this website; people generally do not want to know about technical issues or anything that goes beyond the brochures that come with the glazes they use. Working in the low temperature range has many exciting possibilities as energy costs rise, the DIY movement is best suited to take advantage of these.

Related Information

These Stoneware Potters Do Something Unusual:

They make certain pieces using the low fire process

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Come 6 stoneware is super for most pieces. But there are some things you just cannot do. For example, you can decorate the underside! The one on the right is the back side of the plate. This is Plainsman Snow clay; it has 25% absorption at cone 06-04. This level of porosity is so high that the body can actually absorb the molten glaze if it is melt-fluid enough. However, when fired at cone 06 the body does not absorb any of the glaze. And the plates stay flat when fired on stilts. These are done by the team of Micah & Jeremiah Wassink of Creston, BC (at Pridham Studio). They make matching mugs, but fire those at cone 6 using underglaze decoration with a clear overglaze. But these plates are decorated using a combination of heavily pigmented viscous-melt low-fire glazes and a black underglaze and then finished with a thin layer of transparent glaze. Since the glaze is not crazed, water is not able to penetrate into the body. The layer of clear glaze isolates the heavily pigmented colors from food. Even though they are low fire, these pieces are functional and food safe.

Could low fire be more practical for large decorative plates?

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This 14" plate was made using Plainsman Snow, a dolomite body. It was fired to cone 04 using Spectrum glazes, they were watered down and applied inside and out using a gravity feed sprayer. The high porosity of the body makes the use of these watery gummed glazes possible (older talc bodies would not have worked). Difficult drying and firing make these extremely difficult at stoneware temperatures. But not with this body, it is super plastic yet dries exceptionally well. And it has zero firing shrinkage and can be fired in a fraction of the time! These factors, coupled with the brilliant colors possible at low temperatures, make such bodies an attractive option for potters to make extra money with much less effort. What about fired strength? Large porcelain and stoneware plates are harder but also much more brittle. And they host residual stresses from the drying and firing processes. Pieces made from this body are surprisingly strong. This plate was made by Karel Peeters, Aug 2022.

One way for an ultra clear at low fire:

Magnesia-alkali, low Si:Al ratio, more boron.

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On the left is G2931J, a zinc alkali fluxed and high Si:Al ratio glaze. Those look like micro-bubbles but they are much more likely to be micro-crystals (high-zinc and high-silica is the mechanism for crystalline glazes). G2931K on the right has much more boron, double the Al₂O₃, less SiO₂ and is magnesia-alkali instead of zinc-alkali. It is the product of dozens of tests to find an ultra-clear having a glassy smooth surface. This particular chemistry, although having only a 6:1 SiO₂:Al₂O₃ ratio is ultra-gloss. In addition, it has low expansion, will fast fire and the boron is not high enough to compromise the hardness.

Three low fire bodies need three different clear glazes. Why?

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Glaze fit. The left-most clay mug contains no talc (Plainsman Buffstone), the centre one about 25% talc (L212) and the right one is about 45% talc (L213). Talc raises thermal expansion. The centre glaze is G2931K, it is middle-of-the-road thermal expansion (Insight-live reports it as 7.4) and fits the low-talc bodies (and Zero3 porcelain and stoneware). But it crazes on Buffstone and shivers on L213. The lesson is: Forget about expecting one clear or base glaze to fit all low fire bodies. But there is a solution. I adjusted it to reduce its expansion to work on zero-talc porous bodies and raise it to work on high talc bodies. How? By decreasing and increasing the KNaO (in relation to other fluxes). The three fire crystal clear and work the best in a drop-and-hold firing.

Non-vitreous bodies break very differently than vitreous ones

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The particles in low temperature bodies are not glass-bonded, they only have sinter bonds. Broken edges will only be sharp if there is a glaze. They tend to break off in pieces rather than shatter.

G2931K Zero3 transparent glaze

On Zero3 Fritware Porcelain

This picture has its own page with more detail, click here to see it.

This is an all-fritted version of G2931F Zero3 transparent glaze. I formulated this glaze by calculating what mix of frits must be employed to supply the same chemistry as the G2931F recipe. The mug is made from the Zero3 porcelain body (fired at cone 03) with this glaze. This glaze fits both the porcelain and the Zero3 terra cotta stoneware. The clarity, gloss, fit and durability of this glaze are outstanding.

A non-vitreous body can have a very poor bond with the glaze

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This glaze is on very thick. That gives it the power to impose its thermal expansion (which is different that the body) to the point where it literally flakes off. The problem is worsened at low temperatures when the glaze and body lack fluxes. That means they do not interact, no glassy interface is formed.

Inbound Photo Links

Functional ware at low fire! Don't dismiss it just yet.

Links

Typecodes	Low Temperature Glaze Recipes
Articles	Reducing the Firing Temperature of a Glaze From Cone 10 to 6 Moving a cone 10 high temperature glaze down to cone 5-6 can require major surgery on the recipe or the transplantation of the color and surface mechanisms into a similar cone 6 base glaze.
Recipes	G1916Q - Low Fire Highly-Expansion-Adjustable Transparent An expansion-adjustable cone 04 transparent glaze made using three common Ferro frits (low and high expansion), it produces an easy-to-use slurry.
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.
Glossary	Terra Cotta A type of red firing pottery. Terra cotta clay is available almost everywhere, it is fired at low temperatures. But quality is deceptively difficult to achieve.
Glossary	Cone 6 Also called "middle temperature" by potters, cone 6 (~2200F/1200C) refers to the temperature at which most hobby and pottery stonewares and porcelains are fired.
Glossary	Ceramic Glaze Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
Glossary	Artware Artware, to potters and hobbyists, is pottery made from white-burning clays fired at low temperatures with brightly coloured glazes and decorated using decals, underglazes, lustres, etc.

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