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Reducing the Firing Temperature of a Glaze From Cone 10 to 6

Section: Glazes, Subsection: Adjustment, Adaptation

Description

Moving a cone 10 high temperature glaze down to cone 5-6 requires major surgery on the recipe or the transplantation of the color and surface mechanisms into a proven cone 6 base glaze.

Article

It is amazing how many potters push their clay body or kiln to the limit (e.g. Cone 11) to get a special reactive glaze to melt the way they want. This wastes energy and can produce brittle, bloated ware. It would be rare to find an industrial manufacturer who could afford or would have the desire to do this. It only makes sense to fire to the lowest possible temperature. The energy costs can be much greater than many people realize, it takes much more energy, for example, in a typical poorly insulated pottery kiln to increase the temperature by 1 degree at 1000C than it does to do the same at 100C. In addition, wear and tear on kilns, especially electric, is much less and it is easier to keep the temperature even throughout the kiln at lower temperatures.

At the same time it does not make sense to fire too low either since quality and strength issues come into play below about cone 1. But remember that high temperature firing is not a requirement for strength and quality ware. The range of achievable color is actually better in most cases at lower temperatures. Lower fired bodies can also be very dense and strong. Dental porcelain, albeit an extreme example, completely melts into a white pool of glass far lower than any typical functional porcelain would even begin to vitrify. Admittedly the mullite-glass-silica microstructure characteristic of high temperature porcelains is not as highly developed in middle-fire bodies but with the simple glass-bonding-of-aggregates mechanism you need make no apologies for cone 5-6 porcelain strength.

The question is: can you juggle the amounts of ingredients in a typical cone 10 glaze recipe to make it melt at cone 6? The answer is almost always no. Cone 6 and 10 glazes are two fundamentally different things (in the sense that they employ different fluxes). Moving a glaze from cone 10 to 6 is actually a huge change. Increasing the content of existing fluxes will not be sufficient, you must introduce new more powerful (better melting) ones and learn a new set of dynamics and tradeoffs.

Cone 10 glazes employ raw materials instead of frits and they don't really melt at one temperature, they usually soften over a wide range. Microscopically, raw glaze powders are mixtures of a wide variety of mineral particles that have very different melting temperatures and behaviors. Many particles actually do not melt on their own at normal kiln temperatures but they are dissolved into the melt created by others. Consider an inventory of some of them (we won't look at colorants, opacifiers, variegators):

Pure mineral flux particles like dolomite, whiting and talc are active melters at cone 10 but certainly not at cone 6. You could increase them all you like, you'll never get a workable cone 6 glaze. By ' workable' I mean that although you might be able to get it to melt, so much of the recipe would be taken up by these materials that a stable low expansion glaze could not be created for the lack of alumina and silica).
Kaolin and ball clays are refractory (melt high) and (they have to be dissolved by other things). To reduce a glazes melting range these materials obviously must be reduced to a minimum. However their amounts can only be taken down to about 15% or the glaze slurry won't suspend or dry hard (unless you employ organic binders that introduce side effects not easily dealt with by potters or small operations). More important, clays are the key alumina supplier, taking them too low will detrimentally affect glaze hardness and melt viscosity and increase thermal expansion.
Silica is no where close to melting at stoneware temperatures. It has to be dissolved by other materials that create a fluid melt. Attempting to employ high temperature fluxes at cone 6 is not going to dissolve much badly needed silica into the melt.
Feldspars (and Nepheline Syenite) have complex chemistries and soften over a range of temperatures and they have begun to melt at cone 6. However they don't melt well. If you add enough feldspar to a high fire glaze to melt it at cone 6 you have a guaranteed crazing glaze as well as one that is likely to leach because sodium and potassium will be oversupplied.

The bottom line is that we cannot just reorganize a cone 10 recipe to melt at cone 6. We must add something new, a flux or fluxes not normally found in cone 10 glazes. We must also reduce the proportion of silica and alumina in the glaze, since even with added fluxes it is not possible to get a glaze to dissolve the amounts of high melting alumina and silica typically found in cone 10 glazes. I am not saying it is impossible, you might be the magician who has found a way. But remember, the challenge here is to adjust an existing cone 10 recipe to melt at cone 5- 6 and not craze or leach or scratch easily and still have good application and working properties.

The most obvious solution is to add powerful fluxes like like zinc oxide or lithium carbonate (not commonly used (or needed) at cone 10). They melt very early and vigorously and can impart significant melting effects in small amounts in some circumstances. The only problem is that this is not one of those circumstances. A little zinc is not going to dissolve a lot of refractory particles. If you add enough zinc or lithium to make the glaze melt at cone 6, you will have a whole new animal and a whole new set of problems (especially with regard to color, surface character, tendency to devitrify (crystallize), crawl, bubble, etc.)

I want to point out that when I say we need to 'add something new', I did not mean a material. I meant an oxide. Materials are composed of oxides, and most materials contribute more than one oxide (sometimes 6 or 8 different ones). When you talk about a fired glaze and the way it melts and solidifies, you are talking about chemistry (oxides) whether you like it or not (coincidentally, zinc oxide and lithium carbonate are among the few materials in ceramics that contribute only one oxide to the melt). If we add or remove feldspar, for example, we are making a fundamental change in the balance of oxides in the glaze, that does not jibe with the objective of making changes that will give the greatest reduction in melting behavior accompanied by the least change in overall glaze appearance and fired properties (e.g. color response, thermal expansion, surface character, working properties).

There are two main approaches we can take.

Method 1: Transplant Mechanisms

At the risk of negating most of what I have just said, identify the mechanisms in the glaze and transplant them into a good base glaze for cone 6 that has similar surface texture. Answer these questions:

What gives it the color? Is it a stain, a metal oxide, a stained clay or mineral? Put this material in a cone 6 base. Note however that you need to think about whether a color depends on a sympathetic chemistry in the base glaze. Many stains, for example, require that the base glaze be zinc or magnesia free, for example. Others require the presence of a certain amount of CaO.
Is the glaze opaque or transparent? If it is light in color on the edges then it is likely transparent or partly opaque. If it is a deep and vibrant color it is likely transparent, if it is a pastel color it is likely opaque. If it contains an opacifier like tin oxide or zircopax, then transplant it into the cone 6 recipe.
Is the glaze variegated or mottled? If the effect is a result of the presence of rutile or titanium then transplant either material into the cone 6 glaze.
Does the glaze have speckles that are a product of a granular or unground material? If so transplant these contributors into the new recipe.

There are other factors that are not as easy to transplant. These often relate to visual effects in glazes that are very fluid or runny. For example, glazes that form small or large crystals or a crystal mesh across sections of the surface during cooling need to have very fluid melt. Often matte glazes are actually over-fired cone 6 or 8 glossy glazes that have completely crystallized. For this type of glaze you will need to have an equally fluid melt in the cone 6 range to transplant things into (that means it is going to need to melt well below cone 6, e.g. cone 2). This will not be easy since getting a glaze to melt that low means removing much of the low expansion silica and alumina, that in turn means crazing will occur.

Another mechanism that is not easy to transplant is the character of the surface. Dolomite matte glazes, for example, have a pleasant silky surface that is a product of what CaO and MgO do around cone 10 in the right host glaze. Obviously you cannot transplant this type of mechanism. You must create or find a base that already has the surface you want (or adjust one that is close since matteness is a product of the glazes chemistry, that is, the mix of oxides it contains). Fortunately, at cone 6 alumina, calcia and magnesia mattes are all possible as they are at cone 10.

Method 2: Add the Magic Oxide

The magic oxide to add is boron. The reason boron is such an ideal additive is that it is more than a flux, it is a glass like silica yet it melts low. This is marvelous stuff for this purpose. One of the beauties of boron is that it is compatible with most colorant, matte and variegation mechanisms. But there is another even more important reason: Boron has a low thermal expansion, additions of it will reduce glaze crazing. This is so important because most high temperature glazes contain far more high-expansion Na2O, K2O than can be tolerated at cone 6 without producing crazing. In addition the low-expansion silica and alumina they contain must be reduced to achieve melting at cone 6, this also moves the glaze toward crazing.

The next questions is: What source of boron should you use? The best answer is in the world of frits. There is a boron frit out there to meet almost any fluxing challenge. However each frit contributes other oxides besides boron, that means that to reach the goal of only increasing the boron in a glaze you need to be able to reduce other materials in the recipe that contribute the same oxides as the frit. Thus to do what I am about the describe, you need a ceramic chemistry software program. The kinds of material juggling I describe are dealt with in the examples section of the INSIGHT manual.

The first thing to consider is this: Does the glaze need clay (does it settle or lack dry hardness?) or does it have too much clay already (does it shrink and crack on drying?). If the former, use something like Ferro Frit 3134 (20% CaO 10% Na2O, 23% B2O3, 46% SiO2), . Here is why: Since it contributes Na2O and CaO, that means you will be able to reduce the amount of feldspar in the glaze (because it is normally the major contributor of Na2O). That reduction will also mean a loss of Al2O3, but since the frit contains none you will be able to supply it using a kaolin. Also, you will be able to reduce CaO-contributing whiting or wollastonite if these are present (since the frit contributes CaO). If the glaze already has too much clay, then use Frit 3124, it has almost the same chemistry, but it sources alumina also (meaning you can reduce alumina-contributing clay). If the amount of clay in the glaze is OK, then try Frit 3195, it is balanced enough to be a glaze in itself.

The second last question is: What level should the Al2O3 and SiO2 be at in the newly adjusted glaze? My suggestion is set them to about 2/3 of what they were. However, a rule of thumb is to always have as much alumina and silica as you can in the glaze. Since B2O3 (boron) performs the same function as silica you can afford to have lower SiO2 in the interests of good melting. Also remember that boron has very good alumina-dissolving abilities, thus it is often possible to leave it at the same levels as it was in the cone 10 glaze.

The last question is: How much boron do I add? I would start with about 0.3 molar equivalents. If the glaze is reactive and has to be very runny to achieve the desired effect, you might have to take the boron as high as 0.8.

Some Other Points

If the cone 10 glaze is a stoney matte and is not melting enough, then to duplicate it faithfully I guess you have to make a cone 6 glaze that is not really melting enough either. However this is certainly not advisable for functional ware.

You will want to fire tests to compare the degree if melting (consider using a flow tester. Be sure to try the glaze in thick and thin layers and watch for crazing.

Different minerals release their oxides to the melt at different temperatures, even though they might have similar chemistries ( for example wollastonite and whiting both contribute CaO yet the latter requires a higher temperature to release it to the melt).

Following are some glazes common to cone 10.

White matte: Just opacify a matte base glaze using Zircopax or tin oxide.
Tenmoku: Use high iron in a fluid base. However you will not get the same light brown crystal lines on the edges of contours and it will crystallize a yellowish color if cooled too slowly.
Robins Egg blue: Add a little cobalt and rutile to a matte base.
Black: Add a black stain to a clear glossy glaze.
Purple/Lavender: Use a high magnesia base clear (use Frit 3249 or Fusion F69 to source the MgO, MgO sourcing raw materials won't melt at cone 6) and add cobalt carbonate or oxide (or a blue stain) around 1% and adjust as needed.
Celadon: Add a green celadon stain to your clear base.
Oatmeal/Cream: Add some iron, a light brown stain or rutile (rutile will also variegate it). Add a little Zircopax to opacify if necessary (to bring out the pastel shade).

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Authors

Tony Hansen (Owner)

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