Creating Your Own Budget Glaze
Section: Glazes, Subsection: Formulation
Description
How to take a stockroom full of unused materials and turn them into a good glase. This article helps you appreciate that the glaze formula is the important thing, you can use any number of materials to supply the oxides.
Article
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 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
INSIGHT has a help menu with an option that displays useful limit charts showing the normal range of values for each oxide for different glaze types. Using one of the charts as a target, it is quite simple to determine what oxides need adjustment to make the glaze function well at a specific temperature. Be-low is an example of a limit chart from INSIGHT. The normal limits for cone 6 glazes are shown. The SiO2 can be some-what lower if B2O3 glass is present.
| 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 textbook glazes that almost never work. 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
Forget about those textbook glazes and become "material driven", use economical materials at your easy disposal to create your own glazes.
Authors
- Tony Hansen (Owner)
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