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Formulating Ash and Native-Material Glazes

Section: Glazes, Subsection: Formulation


How to have a volcanic ash analysed and them use ceramic chemistry to create a glaze that contains the maximum possible amount of the ash for the desired effect

Article Text

To most new potters, the idea of creating your own volcanic ash glaze is really quite romantic. Unfortunately, the actual job of trying to produce one that is reasonably functional is not quite so exotic.

Many textbooks supply recipes that employ volcanic ash, but the results of force fitting your ash into these could prove to be a real time waster. Even using a textbook recipe as a simple trial and error starting point, often ends up in blind allies that go nowhere.

The most foolproof way to develop a base glaze is to use calculation, even for native materials like ash. When you view recipes as combinations of oxides, and materials as "sources" of these oxides, suddenly there are an infinite number of material combinations (or recipes) that will yield a target formula. Each of these recipes will fire with subtle differences but in other respects they will be very similar. Having a formula for your ash will thus give it equal status with any other material you have. But where can you obtain the formula for your ash? It is true that some books list a range of ash formulas. "Electric Kiln Pottery" is an example, it has a dizzying list of ash formulas of incredible variation in oxide chemistry. But picking one that closely matches your material is like playing roulette. Don't bother. If you bought the material, I recommend contacting the manufacturer to get the analysis. If not, spend the $50 and have the ash analysed at a lab.

As a demonstration, I dug out a good-sized sample of volcanic ash from a six-inch thick seam at a local clay quarry (vintage Crater Lake volcanic eruption). I mixed the pile thoroughly with a shovel to assure easy sampling of a specimen which was representative of the entire supply. I sent a small amount of the material to a lab for a simple analysis to determine the amounts of each significant ceramic oxide. Finally, I keyed this into INSIGHT and inserted it into the program's materials database, making it available for reference in future recipes entered.

Following is a formula calculation report of the material.

DETAIL PRINT - Oxide Analysis
MATERIAL               PARTS  WEIGHT   CaO*   MgO*   K2O*  Na2O* Fe2O3*   TiO2   P2O5  Al2O3   SiO2
WEIGHT OF EACH OXIDE                   56.1   40.3   94.2   62.0  160.0   79.7  142.0  102.0   60.1
---------------------------------------------------------------------------------------------------  MATERIAL
Expan OF EACH OXIDE                    0.15   0.03   0.33   0.39   0.13   0.14   0.00   0.06   0.04   Cost/kg
---------------------------------------------------------------------------------------------------  -------
 CaO................    8.04   56.10   0.14                                                             0.15
 Na2O...............    0.10   62.00                        0.00                                        0.39
 K2O................    0.30   94.20                 0.00                                               0.33
 TiO2...............    0.04   79.70                                      0.00                          0.14
 Fe2O3..............    1.00  160.00                               0.01                                 0.13
 MgO................    0.72   40.30          0.02                                                      0.03
 SiO2...............   72.84   60.10                                                           1.21     0.04
 Al2O3..............    2.06  102.00                                                    0.02            0.06
 P2O5...............    0.02  142.00                                             0.00                   0.00
---------------------------------------------------------------------------------------------------  -------
TOTAL                  85.12           0.14   0.02   0.00   0.00   0.01   0.00   0.00   0.02   1.21     0.05
UNITY FORMULA                          0.83   0.10   0.02   0.01   0.04   0.00   0.00   0.12   7.04
PER CENT BY WEIGHT                     8.09   0.72   0.30   0.10   1.01   0.04   0.02   2.07  73.27
Cost/kg  0.05
 L.O.I. 14.38
  Si:Al 60.01
 SiB:Al 60.01
  Expan  4.88

To use as much ash in the recipe as possible, I proceeded as follows:

Cone 6 Limit Formulae
CaO 0.1-0.7 Al2O3 0.2-0.35
ZnO -0.2 B2O3 -0.45
BaO -0.3 SiO2 2-3.5
MgO -0.3    
KNaO 0.1-0.5    

Following is the final recipe and its calculation.

INSIGHT's interactive nature and instant calculation made it easy to juggle different amounts of dolomite and kaolin until the formula was right.

MATERIAL               PARTS  WEIGHT    CaO    MgO    K2O   Na2O  Fe2O3   TiO2   P2O5  Al2O3   SiO2
WEIGHT OF EACH OXIDE                   56.1   40.3   94.2   62.0  160.0   79.7  142.0  102.0   60.1
---------------------------------------------------------------------------------------------------  MATERIAL
Expan OF EACH OXIDE                    0.15   0.03   0.33   0.39   0.13   0.14   0.00   0.06   0.04   Cost/kg
---------------------------------------------------------------------------------------------------  -------
 PUMICITE...........   60.00  577.23   0.09   0.01   0.00   0.00   0.00   0.00   0.00   0.01   0.73     0.00
 DOLOMITE...........   20.00  184.00   0.11   0.11                                                      0.00
 KAOLIN.............   20.00  258.14                                                    0.08   0.15     0.24
---------------------------------------------------------------------------------------------------  -------
TOTAL                 100.00           0.20   0.12   0.00   0.00   0.00   0.00   0.00   0.09   0.89     0.05
UNITY FORMULA                          0.20   0.12   0.00   0.00   0.00   0.00   0.00   0.09   0.89
PER CENT BY WEIGHT                    13.85   6.09   0.22   0.07   0.76   0.03   0.02  11.57  67.39
Cost/kg  0.05
  Si:Al  9.89
 SiB:Al  9.89
  Expan  5.50

The whole process took only a couple of minutes. The next step was mixing up the glaze and trying it. I soon found that the ash was very hard, and the glaze had to be ball milled. But once milled, its thixotropic consistency was beautiful and it applied to the ware very well. This was an unexpected bonus! I fired to cone 6, 7, 8, 9, and 10, and found it to be a beautiful crystal tan bamboo at cone 8-9.

When a glaze is completed using this method, it has a balanced formula, one that is less likely to dissolve in acids, scratch on metal, chip on impact, or craze or shiver on heat shock. This volcanic ash glaze is perfectly suitable on the finest dinner ware and I can mix ten gallons for a few dollars! As a balanced base, it is just waiting for some trial and error mixtures with opacifiers, crystal seeders, colorants, speckling agents, etc. While there are many ways to design a glaze, not too many meet with success on the very first trial the way this approach did.

Ball Mill

If you would like to really get into making glazes from your own materials, you will likely need a way to grind them. A ball mill is one of the best investments you can make. It will grind any glaze into a beautifully fine and silky slurry and will make it possible to use almost any material. Milled glazes will melt better and be more consistent and they will have fewer surface defects.

Out Bound Links

By Tony Hansen

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