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27 May 2006
On April 1 2006 a post to Clayart asked if nepheline syenite could be substituted for Kona F4 spar in a cone 6 glaze. Several substitutions were suggested. In an effort to examine how different these might be a ran a test tile of each and one of Kona F4 on its own. As I don't fire at cone 6 I did these tests at cone 10 but hoepfully they will serve to illustrate the point.
The recipes presented were as follows :
| Material | Bryan | Daniel | Ron |
|---|---|---|---|
| Nepheline Syenite | 82 | 75.9 | 77.0 |
| Whiting | 2.1 | ||
| Wollastonite | 3 | 2.5 | |
| EPK | 4.0 | ||
| Alumina Hydrate | 2.3 | ||
| Silica | 15 | 19.7 | 16.5 |
Below are the tests on Glacia porcelain.
FC#38 is just Kona F4.
FC#39 is Ron Roy's sub.
FC#40 is Bryan Johnson's sub.
FC#41 is Daniel Semler's.
Unfortunately I mixed FC #39 and FC #40 a little too thinly. Nonetheless at cone 10 they are all very similar. FC #40 has a nice thick spot in the photo which shows it to be very similar to the others. FC #39 has thicker spots on the edges of the tile which show it also to be very similar. Closeups of each tile are below.
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So the conclusion is that any one of these substitutes would be fine at cone 10 and would likely be so at cone 6. If anyone ever does the cone 6 equivalent tests I'd be very interested in the results.
I have always thought that feldspars produced beautiful crackle patterns
and Kona shows it here well when thick.
The crackle pattern here is much tighter which highlights a thing that
I've noticed before. Craze line density varies not only with the COE
difference between the glaze and the body, but also with the glaze
thickness. Oddly the lines are spaced further apart as the glaze thickens.
I don't know why.
Again a fine craze pattern except where it is thicker.
This one was mixed to about the same consistency as the raw Kona and the crackle is not vastly different, but it is a little.