|Monthly Tech-Tip |
Sieve Analysis ASTM E-11 A-200 A-270 A-400 A-700
(mean % retained on 70 .00 .00
each screen) 100 .01 .01
140 .04 .02
200 .45 .07
270 1.40 .80 .00
325 3.10 1.10 .03
-325 95.00 98.00 99.97 100.00
Sub Sieve Analysis:
% finer than 16 micron 57.0 66.0 88.0
10 micron 37.0 46.0 64.0
5 micron 19.0 25.0 35.0
Property: Test Method Unit Values
Mineral Petrographic Microcline/Albite/Nepheline
Specific Gravity ASTM C128 2.61
Melting Point ASTM C-24 F/C 1868/1020
Bulk Density ASTM C-29 lbs/cu ft 38-55
Feldspars are employed in glaze recipes as melters. So comparing their melt fluidities should be helpful in deciding if one can substitute for another (of course, if possible a soda predominant feldspar should be substituted for another soda spar). Feldspars don't melt alone at cone 6 (2200F) so we mixed each with 15% Ferro Frit 3195. Nepheline Syenite is obviously the champion melter here. Other similar ones can be spotted easily. In the end, degree of melt is a valid consideration in determining if one feldspar is a viable substitute for another in a recipe. Even if the feldspar you want to substitute does not melt as much a little frit can be added to the recipe to make up for the difference (e.g. even just 1 or 2%).
Well, actually, it is not completely pure - I had to mix in 10% bentonite to give it enough shrinkage and leather hard strength to be able to pull itself away from the plaster mold (this bowl was just extracted). It cast without cracks to about 3mm thickness in about 15 minutes. The defloculated slip required more water than normal but it pours and drains beautifully. I have made higher-percentage slurries in the past using Veegum but the casting time was too long. On aging, the previous slurry I made did not change its rheological properties after a month of storage. Nepheline syenite is a fantastic material for ceramics, but it is not used pure like this of course (that being said, firing tests have already shown promising results, with a vitreous product around cone 1).
These SHAB fired bars are 95% nepheline syenite (5% Veegum added). By cone 02 (bar marked #4) it is self-glazing and glass-like with a total shrinkage (plastic to fired) of 15% (less than some porcelains). At cone 03 (the #5 bar) the porosity is 3% (a stoneware). These are actually melted more than normal because the Veegum is a flux.
Feldspars are abundant and varied in nature. They contain small amounts of quartz (while nepheline syenite does not).
About Covia Corp
Covia Corp website
Covia Nepheline Syenite data sheet
About Covia Nepheline Syenite
Nepheline Syenite SDS
The most common source of fluxes for high and medium temperature glazes and bodies.
|By Tony Hansen|
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