Formulating Ash and Native-Material Glazes

Description

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

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:

I consulted limit charts for Orton cone 6 stoneware glazes in the INSIGHT help system. These show typical oxide proportions, providing a balanced target at which to aim. So the task was, clearly, to add the materials necessary to bring the oxide formula of the ash in line with these limits.

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

I wanted the glaze to mature in the cone 7-8 area so this called for aiming Al₂O₃ and SiO₂ content at the high end of the limits.
Looking at the formula of the ash (see "UNITY FORMULA" line), I noted that it was a little high in CaO, short on Al₂O₃ , and high in SiO₂ . It seemed that I needed non-CaO fluxing materials containing no SiO₂ . This eliminated feldspar or nepheline syenite type materials, since their SiO₂ would reduce the amount of ash I could use. Dolomite seemed a logical choice as it was inexpensive and supplied adequate MgO and no SiO₂ . I chose kaolin to supply Al₂O₃ . True, it does yield some unwanted SiO₂ , but much less than a feldspar. A natural clay could have been used here, but again, its higher SiO₂ content would have reduced the ash percentage.

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.

DETAIL PRINT - PUMICITE GLAZE TEST 1
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.

Related Information

Links

Materials	Wood Ash For centuries, pottery glazes have been made from ash mostly mixed with clay and feldspar.
Materials	Volcanic Ash Powdered volcanic ash is used to make ceramic glazes, some varieties have a chemistry similar enough to glaze that high percentages can be employed.
Articles	Ball Milling Glazes, Bodies, Engobes Industries ball mill their glazes, engobes and even bodies as standard practice. Yet few potters even have a ball mill or know what one is.
Articles	Duplicating Albany Slip How Alberta Slip was created by analysing and duplicating the physical and chemical properties of Albany Slip
Glossary	Native Clay A clay that a potter finds, tests and learns to process and use himself. To reduce the costs of importing materials manufacturers, especially in Asia, often develop processes for clays mined in their locality.

By Tony Hansen
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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