|Monthly Tech-Tip |
How to take a stockroom full of unused materials and turn them into a good glaze rather than try umpteen online recipes that require buying yet more materials you do not need and do not work.
You have probably noticed that there are not many rich potters. Money, or the lack of it, is always a major concern at art schools and college ceramics departments. Little wonder then that it is always important to make the most efficient use of equipment and materials at hand. Glaze materials, as everyone knows, can be very expensive. Do the expensive materials produce better glazes? Not always. Often the key is the engineering that goes into the formulation.
I participated in a fascinating example of saving some money, and would like to relate it to you. Although this is very "small scale", I'm using it to demonstrate principles applicable t any operation. Many companies have hundreds of tons of materials they would love to be able to use up in products. For others, the amounts are modest but the challenge is there, especially if there are quite a few different material types.
A new instructor took over a long neglected school ceramics program. She was on a tight budget but inherited a small inventory of assorted glaze materials to work with. She secured suitable glaze recipes but found little commonality between the materials called for in these recipes and the ones on hand. Thus it appeared that mixing the new recipes would actually increase the existing inventory of raw materials. Such a cost seemed unjustified and wasteful.
Then there is the perennial problem of textbook glazes being poor travelers. After all, circumstances vary. Most potters, for instance, will confirm that they have to test hundreds of recipes to find one whose touchy nature they can just tolerate. Likewise, in industries that mix their own glazes, those which function well in one plant produce problems in others. So many technicians have found it easier to just design a glaze from scratch.
What's the best approach? You guessed it: Calculation followed by 'trial and error work' to refine a final recipe. With a little simple guidance, it is really quite easy to create a working glaze after two or three trials! Getting back to our example; here is the inventory of materials that was on hand at the school.
The Inventory of Materials ZINC OXIDE 100 gs CALCIUM BORATE 1 kg CUSTER FELDSPAR 500 gs KAOLIN 2 kg SILICA 2 kg NEPHELINE SYENITE 500 gs WHITING 2 kg DOLOMITE 2 kg BALL CLAY 2 kg
I decided it was best to create a clear base glaze, which could be opacified to make white or colored with stains and oxides to make colored glazes. A large powdered batch could be mixed and easily blended with colorants. So I threw all the remaining materials together into one mix! No, not literally, but in Digitalfire Desktop INSIGHT.
Here is the result.
CUSTER FELDSPAR..... .5 KAOLIN.............. 2.0 SILICA.............. 2.0 NEPHELINE SYENITE... .5 ZINC OXIDE.......... .1 GERSTLEY BORATE..... 1.0 WHITING............. 2.0 DOLOMITE............ 2.0 BALL CLAY........... 2.0 ---- 12.1 FORMULA & ANALYSIS ------------------ *CaO .68 20.04% *MgO .22 4.65% *K2O .02 .96% *Na2O .05 1.74% *ZnO .02 1.05% *Fe2O3 .00 .18% *TiO2 .01 .32% B2O3 .15 5.48% Al2O3 .32 17.11% SiO2 1.53 48.48% RATIO 4.82
Check the links below to learn more about limit (or target) formulas. Using one of the charts as a target, it is quite simple to determine what oxides need adjustment to make the glaze melt better at a specific temperature. Below is an example of a limit chart. The normal limits for cone 6 glazes are shown (if B2O3 glass is glazes melt alot better and Al2O3 and SiO2 can be higher).
|Temp C||880||980||1080||1180||1280||Oxide Cone||012||08-05||04-02||3-7||8-10||CaO||.15-.5||.15-.5||.3-.6||.3-.6||.35-.7||ZnO||-.05||.05-.15||.1-.15||.1-.25||-.3||BaO||-.1||.1-.2||.1-.2||.1-.3||-.3||MgO||-.1||.075-.15||.1-.15||.1-.2||-.35||KNaO (Alkalies)||.35-.5||.35-.5||.3-.5||.2-.5||.2-.45||B2O3||.8-1.5||.6-1.0||.5-.85||.3-.5||-.3||Al2O3||.1-.15||.15-.25||.15-.3||.2-.35||.3-.55||SiO2||1.25-2||1.5-2.5||1.75-3||2.5-3.5||3-5|
The glaze already proposed has a nice selection of fluxes and appears good, except for a little too much CaO and a lack of SiO2 . Here is a recalculation, after the addition of enough silica to bring the SiO2 content up past the 2.5 minimum.
CUSTER FELDSPAR..... .5 KAOLIN.............. 2.0 SILICA.............. 6.0 NEPHELINE SYENITE... .5 ZINC OXIDE.......... .1 GERSTLEY BORATE..... 1.0 WHITING............. 2.0 DOLOMITE............ 2.0 BALL CLAY........... 2.0 FORMULA & ANALYSIS ================== *CaO .68 14.11% *MgO .22 3.27% *K2O .02 .68% *Na2O .05 1.22% *ZnO .02 .74% *Fe2O3 .00 .13% *TiO2 .01 .22% B2O3 .15 3.86% Al2O3 .32 12.05% SiO2 2.85 63.71% RATIO 8.99 WEIGHT 268.55
This brings the ratio up to 9, giving a glaze which should not be too matte or too glossy. Since there is such a variety of other fluxes, I left the CaO alone. The formula is now quite typical of a cone 6 glaze and further tests indicated that, yes, it does perform quite nicely as a silky matte at cone 6 and as a glossy at cone 7 and 8.
So, all the school had to do was buy 4 kilograms of inexpensive silica It resulted in enough to make more than 5 gallons of glaze, leaving money in the budget for extra clay for the children so they could use up all that glaze.
Most people would never try an approach like this. But I have outlined this simple experience to demonstrate a principle that has merit: It is often quicker, cheaper, and easier to design a glaze from scratch than test online undocumented glaze recipe that usually do not work (because of a variety of factors in your studio that differ from that of the author of the glaze). The ability to mix materials and predict the fired product by quick calculation is a big factor in being able to do this. Although this situation could have been more complicated and more materials might have been needed to produce a balanced oxide formula, the principles would have been the same. However, if there is a wide selection of materials, you will find this method will frequently succeed as it has here, requiring the purchase of only one or two materials.
Recipes show us the materials in a glaze but formulas list oxide molecules and their comparative quantities. Oxides construct the fired glass. The kiln de-constructs ceramic materials to get the oxides, discards the carbon, sulfur, etc. and builds the glass from the rest. You can view glazes as recipes-of-materials or as formulas-of-oxides. Why use formulas? Because there is a direct relationship between the properties a fired glaze has (e.g. melting range, gloss, thermal expansion, hardness, durability, color response, etc) and the oxides it contains (links between firing and recipe are much less direct). There are 8-10 oxides to know about (vs. hundreds of materials). From the formula viewpoint materials are sources-of-oxides. While there are other factors besides pure chemistry that determine how a glaze fires, none is as important. Insight-live automatically shows you the formulas of your recipes and enables comparing them side-by-side. Click the "Target Formula" link (on this post at digitalfire.com) to see what each oxide does.
A way of establishing guideline for each oxide in the chemistry for different ceramic glaze types. Understanding the roles of each oxide and the limits of this approach are a key to effectively using these guidelines.
Limit Formulas and Target Formulas
Glaze chemistries for each type of glaze have a typical look to them that enables us to spot ones that are non-typical. Limit and target formulas are useful to us if we keep in perspective their proper use.