Dave Finkelnburg on fri 15 apr 05
Darvan 7 and glazes for brushing
Paulette,
You ask good questions! :-)
Glaze suspensions are somewhat complex systems, with more than one thing affecting the rheology. I think most of what you see is influenced by the amount of dissolved ions in the glaze. Also, at 8 mls in 350 grams you are way overdosing your glaze with Darvan.
Darvans come in different strengths. Darvan 811 is about 40 to 45% NaPAA. Darvan 7 is 25% NaPMAA. There is a feeling in industry that 811 is better for clay systems. However, the literature does not support this. PMAA is reported to be as effective as PAA as a dispersant. What is obvious is if you add a gram or a cubic centimeter of 811, you get more dispersant, and that may be misleading.
The sodium in both these Darvans dissolves in your glaze and increases the ionic strength of the suspension. This goes along with the ions from other sources. When you get too many ions, you pass the Critical Coagulation Constant (CCC) and the glaze sets up. So, the Darvan
that disperses your suspension, contributes cations (Na+) that can coagulate the same suspension!
The real contributors of cations, though, are the Gerstley borate and the soda spar. Both contain some soluble sodium. You may want to consider, at least in part, a less-soluble source of sodium for your glazes.
All the Darvans create a minimum viscosity and then, as you add more, the viscosity of the glaze or slip rises again. You have to watch carefully to detect the minimum. If you are past that, just adding more gets you closer to the CCC and a coagulated glaze.
I hope this is helpful.
Dave Finkelnburg
Paulette Carr wrote:
I have recently experienced some difficulty with two glazes that I
prepare for brushing. One of the glazes contains about 7.5% Gerstley
Borate (glaze 1), and the other glaze contains roughly 50% F-4 Feldspar
(glaze 2).
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