Tasks Accomplished previously:
The last firing, cone 04, resulted in a sugary coating like an under-fired glaze. I don't have a photo.
Tasks to accomplish:
1. increase glass at cone 04.
2. develop colour.
3. measure linear shrinkage.
References: Alan Caiger-Smith, Robin Hopper, John Britt, Amy Waller
Observations & Reflections
I left the test bar for paste 'B1' (Fig.1) to dry for two to three weeks, much longer than I normally would. I noticed shortly before firing that the efflorescence seemed thicker than usual, and more blue than usual. I will experiment with drying time.
The test bar for B1 resulted in a deep, intense copper blue. I quite like it. You can see on the bead (Fig.1) that the oxide percentage may be a little too high; it's flat black in places. I used 3.5% copper carbonate; however, drying time seems to be a factor that increases the saturation of the colorant. The crackle pattern is appropriate for a decorative object, providing an engaging break-up of light. There is the possibility of staining the crackle. The surface is still filled with micro-discontinuities, which a soak would likely cure, or going up one cone, to 03.
The test bar is tricky to read for shrinkage. Shrinkage bars are fired without glaze, so it's easy to read the measure. But here there is no "without glaze", so it is a bit tricky to read the measure. It looks to be 14% shrinkage which seems high. I will have to make better marks.
Workability is not awful, but I did not push it. You can see in the pic below that it takes a ragged edge which means relatively low plasticity and limited workability. It is also somewhat friable when dry. I think it needs some bones.
Robin Hopper's Egyptian Paste ^08
Ball clay 5
Sodium bicarbonate 5
Soda Ash 5
Calcium carbonate 4
Fine white sand 7
Flux: 50, Silica: 18, Clay 23.5
John Britt's Egyptian Paste ^06:
Soda feldspar 38
Ball clay 12
Soda ash 6
Sodium bicarbonate 6
Flux: 50, Silica: 38, Clay: 12
I decided to fire toward the upper end of the Egyptian paste range, about ^04. If you look at Hopper's slides, his glaze is sugary, not glassy, and it has large visible discontinuities. Britt's on the other hand is closer to what I want, a more continuous glass surface. I don't see a reason for Kaolin, it will make it more refractory, and I would in any case choose a more plastic clay, keeping in mind that its iron content must be as low as possible.
So I came up with the following mix, B1 (fig.1), as a starting point:
Neph Sy 30.5
Soda Ash (lite) 10.5
Silica (400M). 30.5
Vee Gum T 3.5
Flux: 45.3, Silica: 30.5, Clay: 21
It combines aspects of both Britt's and Hopper's formulae. B1 is much higher in silica like Britt, and much higher in clay, like Hopper. I think clay is going to be important for a decent range in workability, and some bones to help knit the body together. Temperature at 04 seems fine. Could use a longer soak.
1. Increased the glassiness of the surface
2. Developed an intense turquoise blue
3. Shrinkage is probably at 14%
Task to accomplish:
1. Increase plasticity
2. Decrease friability
3. Test for different drying times: 1 week, 2 weeks, etc
4. Decrease soda ash
5. Redo shrink bar (use calipers)
6. Do an absorption test
BlogArgilla's first posts are about my research into making an Egyptian paste, which is a self-glazing compound. Amy Waller's site is a very good primer on the subject, and has some great links. Egyptian paste is not suitable for food; it is weak, porous, and is used for making symbolic and decorative objects.
The first question might be, why bother with combining glaze and clay into one compound? The short answer is that it bypasses the potter's Sisyphean routine of firing everything twice. As if once wasn't enough. This opens opportunities to produce work in small spaces, without the investment of a large kiln. It allows the making to be less technical, and therefore more people can avail themselves of it. It also lays the ground for the possibility of developing a slip-glaze, i.e. a compound that applies like a slip, and fires like a glaze, great for sgraffito.
Sometimes Egyptian paste is called Egyptian Faience; let's not get stuck on colonialist labels for now. What we are talking about here is a composed paste/clay with a fair range of variations in its ingredients and in their composition, and none of them have anything to do with Faience. The earth in the region of the Levant and other nearby areas is very highly saline. For example, in the Red Sea, the saltiest body of water on earth, one is buoyed on the surface, never sinking, there is so much salt in the water. Salt is a flux, and if there is enough of it in a paste or clay body it will effloresce, i.e. migrate, to the surface as the paste dries. When fired, this migrated salt can help form a glassy surface.
I will return to the subject of efflorescence, because it is a key mechanism of this self-glazing paste. Efflorescence, from the French effleurer, or to flower, is an aptly descriptive term. We shall look at how to encourage the flowering of salt with its pros and cons.