NECESSITY IS THE MOTHER OF INVENTION

Leonard Smith

Necessity is the mother of invention and in the case of the glaze I'm about to describe it surely was. My very first kiln was a beast, fired with a very powerful oil burner. The burner was a self contained unit with a high pressure pump, fan and auto-ignition system. It was so efficient that it used to melt the high alumina bricks in my bag wall until I designed an elongated fire box for it. The problem was that as the kiln was cross draught in design and the heat so intense on the burner side the temperature inside was very uneven with cone 11 at the top and cone 6 at the bottom.

The "necessity" was to develop a glaze that had a cone 6 to cone 11 firing range. I had long heard of the Calcium Alumina Silica Eutectic which melted at 1170oC so I decided to work on a recipe based on this formulae. I found the recipe in Ceramic Glazes by C W Parmelee and using unity formulae in my glaze calculation software I worked out the recipe from the formulae.

What followed was a long series of testing and development. The oxide recipe for the Calcium Alumina Silica Eutectics is  

Recipe Percentage

Calcium Oxide 23.25

Alimina 14.67

Silica 62.08

Total  100.00

Unity Formula for Calcium Alumina Eutectic:

Si:Al Ratio= 7.2:1 Percentage Analysis:

Using the unity formulae I worked out the recipe using common minerals.  

Recipe Percentage

Whiting 33.36

Kaolin 31.30
Silica 35.34

Total 100.00

As is obvious this introduced some impurities when compared with the relative purity of the original formulae but these were considered minor so I used this recipe as my starting point. No matter how much theory you have there is simply no substitute for the empirical method, ie suck it and see, or in this case melt it and see. So I fired samples all over the kiln and as was expected it was a very good clear glossy glaze at cone 6 but quite runny at cone 10.

The next series of tests were purely intuitive adding Potash Feldspar for its ability to stiffen the glaze whilst maintaining a wide firing range and adding Magnesium in the form of Talc to give both durability and surface quality to the glaze. After this series of tests I settled on a recipe that, fortuitously, not only had a very wide firing range but several other desirable qualities as well. For me high clay content glazes are superior production glazes, they suspend well in the bucket, they apply well to the bisque ( in this case the glaze was an excellent raw glaze) and they are very tough once they dry on the pot so eliminating many handling faults.

This glaze also has excellent glaze fit, crazing only on the most siliceous bodies and so by adds considerably to the strength of the body it covers. In fact in its Tenmoku/Teadustversion it was prone to shelling and shattering if I didn't crash cool the kiln to decrease the development of body cristobolite. The final glaze is clear without any additions, a good Celadon with 2% Iron. It varies from a soft semi matt at cone 6 to a high gloss at cone 11. Best of all it never runs, and for this reason alone I have introduced it as a standard glaze of all the colleges I have worked at and gave it away at the many workshops I did over the last 30 years. Because the glaze is so unusually with its high quantities of clay and silica, it is very easy to identify it in its many variations and I have come across it in many locations and wish that had thought to patent it, I would probably be rich now.

LEO'S CLEAR GLAZE

Cone: 6 to 10 Colour: Transparent Clear Surface Texture: Semi Matt to Shiny/Glossy Firing: Ox. or Red. Glaze type: High Calcium  Recipe:  

Potash Feldspar 15  

Whiting 19  

Talc 7  

Ekalite 28  

Silica 30  

Possible Health Hazards: Talc: wear an approved dust mask when handling dry material Silica: free silica-wear an approved dust mask when handling dry material Unity Formula for LEO's CLEAR GLAZE Si:Al Ratio= 7.6:1 Percentage Analysis: ------------------------------------------------------------------------ ® Leonard Smith

(1) Ceramic Glazes by Cullen W Parmelee Third edition revised by C G Harman, CBI Publishing Co. Boston, 1973. Page 289. (2) HyperGlaze IIx by Richard Burkett & Matrix Beta6 by Lawrence Ewing both reviewed in earlier PIAs.