✰ CONVERSION OF A CONE 9 GLAZE TO A CONE 6 GLAZE ✰ by Val Cushing
If you sometimes wonder why you need to know any glaze chemistry, one answer I would give is that it might help you solve the practical problem of adjusting the melting point of glazes from one firing range to another. It can help you in other ways too, but knowing how to bring cone 9 glazes down to cone 6 is useful and practical. There are many was to do this. If you have the skills and information to do it the most accurate way would be to work your original cone 9 glaze (100% batch formula) back to a unity molecular formula (UMF). You would then alter the UMF to work at a lower temperature (if you know how) and then recalculate the glaze to a new 100% batch formula. There are simpler ways to do this. Most of us work directly with materials, rather than through the glaze calculation system. Working accurately with materials requires knowledge, experience and some luck. Working without knowledge and experience is also possible, especially if you have lots of luck!
I am going to give you eight alternatives that will bring cone 9 glazes to cone 6. Many years ago I wrote an article for Studio Potter promoting the idea of firing to cone 6 in electric kilns. I won’t repeat all of that here. Two obvious advantages are that electric kilns can be used almost anywhere with a few basic precautions and that cone 6 saves money and is more ecological than fuel type kilns. I contend that there is no real reason to fire higher than cone 6 when firing in oxidation. You can do everything at that temperature that you can do at cone 9. In fact, you can do more because at cone 6 you can get many of the advantages of earthenware with its stronger, brighter colors as well as all the characteristics of stoneware and high fire.
Some glazes will look “juicier” at cone 7 or cone 8 but if they look a little dry at cone 6, simply add more fluxes (as described below) to cause a bit more melting. Don’t be afraid of glazes that are called cone 9 reduction or even cone 6 reduction. They will work in oxidation, too in most cases. The base glaze (a glazer with out colorants) will work in oxidation or reduction. There may be some changes – for example, reduction satin and matt glazes will probably look a bit glossier in oxidation, etc., but that can easily be altered.
Please take ht following suggestions as simple guidelines. Make some tests and see what happens. I always recommend that glazes be developed as base glazes first. Color tests follow once you have established the surface texture (glossy, satin or matt) and the light transmission (transparent, semi-opaque or opaque).
To bring a C/9 glaze down to C/6, you can begin with one or more of the following alternatives. Keep in mind that a glaze which is already glossy at cone 9 will need less flux to bring it to cone 6, while a glaze which is matt at cone 9 will need more flux at cone 6.
Alternative 1 – try switching the feldspar to either nepheline syenite or Kona F-4. They are both lower melting spars than the potash spars. That may be all you need to do! Nepheline syenite is the lowest melting feldspar.
Alternative 2 – try adding GERSTLEY BORATE to your existing 100% batch formula base glaze. Add gerstley borate in increments of 5% starting at 10%. If you want to see the whole range, try 10%, 15%, 20%, 25% and 30% (all added to the existing 100%).
Try adding FRIT to your base glaze in the same manner described above for gerstley borate. Frit is a kind of “magic bullet” for dropping cone 9 glazes down to done 6. Frits are premelted glazes ground up into powder. Most frits Most frits melt into glazes at cone 04. Obviously they will be very strong melting materials at cone 6. They will bring any cone 9 glaze to cone 6. Frits have different formulas, I would recommend that you consider using one or all of the four frits mentioned below. They melt easily and will promote interesting colors.
A look at the Unity Molecular Formulas of Frits section of my Handbook will tell you what the frit of your choice is composed. Analyzing all this may tell you what color effects to expect. This same section also lists frits from several companies, which are comparable so if one frit isn’t available to you another comparable one may be.
Alternative 3 – Ferro frit 3124, very high calcium, high boron – add 10%-30%
Alternative 4 – Ferro frit 3134, very strong, high boron, high calcium, no alumina – add 10%-30%
Alternative 5 – Ferro Frit 3185, high sodium, high boron, no alumina – add 10%-30%
Alternative 6 – Ferro frit CC 257-2, high barium with TiO2 and ZrO2 – add 10%-30%
Alternative 7 – try adding LITHIUM CARBONATE, a very strong flux – 5%, 10%, 15% to you 100% base glaze.
Alternative 8 – If your cone 9 glaze has between 16% to 30% clay in it, try lowering t he clay to about half the amount you took of the clay to the feldspar to bring glaze total back to 100%. This adds more flux.
Once you have established the kind of base glaze you want, the search for interesting colors and textures should begin. Keep in mind that whatever you have done to alter the cone 9 glaze to cone 6 will also affect the colors that it will produce. It isn’t the same glaze anymore. The addition of things like gerstley borate, frits or lithium carbonate, etc. will change the colors that the base glaze will produce. The main difference will come from the additional fluxes containing B2O3 which all the frits mentioned contain as does gerstley borate.
The only way to preserve the exact character of a cone 9 glaze at cone 6 is to bring the cone 9 batch recipe to a unity molecular formula. Then lower the alumina and silica slightly keeping the ratio between them the same and make a slight increase in the alkaline oxides and in the calcium. Do not introduce any new oxides which are not already present in the UMF. In this way you will retain the compositional integrity of the glaze and hopefully its appearance, while lowering its melting point to cone 6. To do this, use the Handbook Chart for Calculating a Unity Molecular Formula from a Batch/Parts by Weight Glaze Recipe.
Following my suggestions will in most cases add more calcium and boron and more lithium in one case, to your base glaze. A look at the Unity Molecular Formulas of Frits section will tell you the composition of the frit you are using. Analyzing all this may tell you what color effects to expect. This same section also lists frits from several companies which are comparable so if one frit isn’t available to you, another comparable one may be.
If you are going to proceed with these suggestions, you may be asking yourself which alternative to try first or if you choose a frit, for example, how much will you add to alter the base glaze? etc. It all depends on how serious you are and how much testing you are willing to do. When you test glazes, you should use bisque fired test tiles made from the clay body you are using. Vertical tiles are better than flat tiles. I take a fat coil or clay about 4” long and squeeze it in my hand so that it conforms to the shapes of my fingers. I then flatten out the base to a circular disc about the size of a silver dollar. I use the flat base to write on with a black underglaze pencil to identify the tile. These “glaze mice” work very well and are quick to make. However you make the tiles, be sure to make enough, probably 20 or 30 will do.
One way to start might be to do one test for each of the alternatives I have suggested. Try switching to nepheline syenite (alternative 1) if that’s appropriate. Try, say, a 15% addition os gerstley borate and of each of the other flux additions suggested. That would mean doing only seven or eight tests to begin with and should give you the information you need to go on to really nail it down to what you want.
Naturally the more tests you do and the more extensive they are, the more help you will get. A more focussed test series would be to begin with 10% of any one of the frits, or other flux additions and increase the percentage by 2% and run it all the way to 30%. So you would have 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28% and 30%. If you did those eleven tests with any one of the suggested additions, I would guarantee that somewhere in that run you will find excellent cone 6 glazes. If you did a test like with all the flux suggestions, you would probably have more glazes working at cone 6 that you would know what to do with. When you really get into this and things start working, doing glaze testing is like eating potato chips - it’s hard to stop!
There are quick ways to f running tests like the 2% addition test described above. For example, once a 50 or 100 gram dry batch of glaze has 10%frit added and is mixed with water and put on a test tile, you can then add 2% more frit to this same batch to test 12%; add another 2% more to test 14%. Even though your wet batch is obviously a little less each time you put some on a test tile, the results will still be good information without having to use a new batch for each and every percentage tested. Also when testing. it takes no longer to weigh out a couple hundred grams of your base than it does to weigh out just 100 grams. Weigh out the base, mix it very well and then divide it into dry test batches.
When you fin a base you like, re percent the glaze including your new ingredients back to 100%. (to do this , add up the total of the column of numbers. Then divide each number in the column by this total and multiply each answer by 100).
Another “magic bullet” for texture and action isn a glaze is TiO2, titanium dioxide. Add 6% to any base glaze as an additive below the line, i.e., do not put it in the 100% formula of the base glaze.
Once you have a base glaze with a texture you like, run color tests. Start with 2% of copper oxide, cobalt carbonate, red iron oxide, etc. (See the Handbook section for Running Color Tests on Base Glaze).
Alternative 9 - Simply take a cone 6 base glaze which looks promising and try some color tests. Some good ones to start with from this book would be V.C. “D” Base - if it’s a bit stony, ad 10%frit; V.C. Pam Fredericks and V.C. Karen Starshine are bases which will give a lot of “action” and texture. Other possibilities are D.K. 11 Revised and K.S. HU 4.