A Cone 6 Glossy Base Glaze is Born

I love to use the Glaze Dragon as a symbol of the attitudes that prevent us from getting control of our glazes. It is a good analogy for the prevailing 'glaze recipe culture' and our addiction to the 'roulette wheel' approach to finding glazes. The dragon wants us to think glaze science is a 'mystery', that existing recipes were dropped from heaven and questioning or adjusting them is blasphemy. The advent of the internet has actually made us worse, we're furiously trafficking in more recipes that don't travel well.

Decades of glossy magazines have taught us that 'appearance is everything'. Schools still crank out generations of ceramists with numbed consciences regarding their functional accountability to customers. At conferences we've argued about craft and art and addressed the politics of ceramics. But the closest we ever seem to come to understanding glazes is learning application methods. It is true, many authors and teachers have been singing the 'learn-your-materials' song. But those studying materials feel ill-equipped to create balanced, fitted, strong, color compatible glazes, opening the kiln is still like pulling the handle on a slot machine.

The key to creating good recipes is the marriage of material and oxide knowledge. Oxides? Fired glazes are built out of them. They are a universal language of glazes. The kiln decomposes ceramic materials into their oxide building blocks and reassembles them into the fired glaze. Oxide know-how has been out there all along and computer users have rediscovered it. These people have not been on the roller coaster, they have been making really exciting things happen in the studio and on the factory floor, learning from their mistakes as never before. And the Internet has allowed them to find each other and rejoice in not being as alone as they thought.

But many questions still remain. The most asked question I hear is: "Do you have a good cone 6 glaze?" It is asked so often because so many non-satisfactory answers have been given in the past. What we need to answer it well is a ‘attitude transplant’ about what a good glaze is and how to get it. I would like to demonstrate how we can view materials as both powders with a physical presence that shapes the properties of the glaze slurry and also as ‘oxide warehouses’ that supply building blocks to the fired glass.

What is a Good Recipe?

After enough failed 'affairs' with textbook or internet recipes you can lose your ability to recognize true glaze quality. It is much more than simple visual character. A good recipe is:

• Reliable, stable, and predictable. Its chemistry is balanced so that firing it a little slower or faster, lower or higher, applied thicker or thinner makes minimal difference and still is predictable. It offers the true economy of quality materials.
• It works with a variety of stains and has associated documentation about what colors it does not work with, why, and what to do about it.
• It has maximum resistance to leaching so you don't poison your customers.
• It applies evenly without drips or cracking even on very dense bisque ware and fires without a tendency to blister or crawl. It does not settle hard in the bucket and it does not gel or powder off excessively.
• It fires to a hard and durable surface.
• It does not pinhole, crawl, or blister.
• Poor glaze fit dramatically weakens ceramic ware. Thus it would have a relatively low thermal expansion so it not only does not craze on the intended body(s) but fits well so it strengthens ware.
• It offers good transparency not only for its own use but as a clean starting point for other glazes.
• It employs easy-to-get materials whose chemistry is published by the manufacturer or distributor(s).
• You can easily adjust its melting temperature, expansion, gloss, etc.

Rationalizing a Cone 6 Glaze

How many textbook glazes have you found that could withstand this scrutiny? I've had ones that looked good but satisfied almost none of these requirements! Remember a fundamental point: The key to creating good recipes is the marriage of material and oxide knowledge. It would be very difficult indeed to meet the above requirements using a material-blending approach. It would be impossible to chance upon them by mixing recipes! It is imperative to look at the fired glaze as a formula. What is a formula? A formula compares the numbers of oxide molecules in the fired glaze. Oxides tend to have predictable effects.

You might respond: "That's too much trouble, it is all a matter of faith and trust anyway, just give me the glaze recipe!" But the Potter's Prayer is not enough, ethics and accountability are involved. Other's materials are different than mine, as is their clay, process, firing, taste, and type of ware. Someone's poor implementation or misapplication of my recipe could produce weak, difficult-to-clean, crazed, shivered, leached ware that hurts the reputation of the ceramic community, could injure people physically, and makes me partly accountable for the technical quality of their ware! Ethically I should understand and document a recipe before handing it out to prove that I am conscientious and willing to field questions to help users adapt. Borrowing recipes from others without being willing to adjust or question them is looking more and more like taking someone else’s prescription.

A New Glaze is Born

Let's try to create and test glaze recipe that addresses the above requirements. I'll leave the trial and error additions of colorants, opacifiers, specking agents and variegators to you. First, consider this limit chart for cone 6 non-leaded stoneware glazes.

This chart is not a boundary waiting to be proven invalid, it is simply a recommendation of the ceramic industry. Formulas within these ranges present the least problems and tend to test well for leaching and hardness.

Now lets talk about the oxides in the above limit chart. As we do so don't think about the powdered materials you use to mix up the glaze batch, imagine being able to look at the fired glaze under a microscope powerful enough to reveal the oxide structure. I will consider them in the order they can most easily be supplied from parent materials.

The Calculations

Classic ceramic calculations that have been around since early this century but we just never bothered. Now computer software makes it easy to do them and we are figuring out new ways to apply them. Shown here is an example of the 'business end' of a ceramic calculation computer program.

After supplying each of the above oxides from the indicated materials I juggled material amounts to achieve 5% roundoffs. This held all the oxide amounts fairly close to target (except that CaO is a little over the target).

Here is the recipe:

G1214W CLEAR LOW EXPANSION
WOLLASTONITE     10.00  CaO    0.57*
FRIT 3134        25.00  K2O    0.02*
PIONEER KAOLIN   25.00  Na2O   0.17*
SILICA           25.00  Al2O3  0.35
F-4 FELDSPAR     15.00  B2O3   0.24*
                        SiO2   2.94

For a web page showing this recipe with Hyperlinked materials and oxides (i.e. if you need to find substitute materials in your area), please visit http://digitalfire.com/recipes. To research materials and oxides use http://www.ceramicmaterials.info.

Now please remember, I am not promoting this recipe as the end-all of cone 6 functional base glazes. This one is part of a larger project centered on the G1214M recipe. I am promoting a way of thinking. Take this recipe, understand it, adjust it, make it work for you. For example:

• Add more SiO2 or Al2O3 if it melts well for you, reduce if it does not.
• Reduce the B2O3 for CaO if you need to use chrome-tin colors.
• Reduce the SiO2 and increase the CaO to get a matte.
• Add opacifiers, colorants, variegators, specking agents to create an infinite number of effects.

By the way, I found that this recipe worked well with equal parts by weight of water and powder to give a specific gravity of 1.45.

Testing Your Glaze

Mention the idea of testing a glaze and 99% of us think in terms of dipping a test tile and firing it to see how it looks. But testing should consider far more. You should test slurry properties, application properties, devitrification, clarity, crawling, stain compatibility, leaching, hardness, cutlery marking, crazing/shivering, reaction with solubles, melt flow, firing volatility, and for general functional use.

Oxide Knowledge, Electronic Textbooks and the Net

It is true, oxides and formulas can seem intimidating at first. But consider: I can list the oxides on the fingers of my hands and they have predictable behavior. Compare this with thousands of materials whose effects in recipes are much more difficult to predict (there is no such things a ‘material limit recipes’). Perhaps you wonder where a person might learn about this 'predictable behavior' of oxides. It does tend to be pretty sparse in textbooks. The website at http://www.ceramicmaterials.info is the best place.

Electronic books are also available on the Internet and they are becoming more and more common. No one likes to read a book on a computer screen but still there are some compelling advantages that are hard to ignore. These books are searchable by the word, they can have clickable links in the index, table of contents, etc, you can print out any range of pages with very good fidelity to the original, publishers can deliver books via the Internet and protect them by password, electronic books can contain vibrant color at no extra cost, they can be updated easily and often. Shown here, for example, is the Magic of Fire Reference book (available at http://digitalfire.com/magic) as you would see it in the Adobe Acrobat reader program on Windows, Unix, or Macintosh. You turn the pages like any book but you can also zoom in and out, search, show four pages at once, print pages, etc. Once you have a book like this installed on your computer it is only a click or two away. This is the future of technical publishing.

Crazing

It is possible that you need to move still further to stop crazing on your clay body, especially if it is very low in silica. We suggest increasing B2O3 slightly and then increasing Al2O3. Or you may simply be able to add silica. In serious cases of crazing the magic oxide is MgO, increasing at at the expense of K2O and Na2O will dramatically reduce the expansion. MgO does not melt as well at middle temperature but we have found the diversifying the fluxes and increasing the boron a little will make room for as much MgO as you need. Here is our G1215M recipe. Although the materials in the recipe are quite different, the chemistry is very similar. The small amount of zinc should not be a problem for most, but if it is you can leave it out and see if it still melts enough, if not add a little more frit. You can try increasing the kaolin further to drop expansion. Note that this recipe does not total 100.

Silica                 35.0
Strontium Carbonate     2.0
Zinc Oxide              2.0
Spodumene              10.0
Dolomite               11.0
Frit 3195              30.0
EPK                    20.0

As of Dec 2002 we have released a new low expansion base glaze numbered G1215U. It turns out that substituting MgO for other fluxes is by far the most effective way to reduce glaze thermal expansion. Fusion frit F69 or Ferro 3249 are the magic behind this type of approach (Frit 3249 contains a little CaO in addition to the MgO). These high magnesia frits melt much better than any natural MgO source and they do not produce gases during decomposition. Here is the recipes we are working with now:

G1215U
PIONEER KAOLIN.. 14.0
SILICA.......... 26.0
WOLLASTONITE.... 14.5
F-4 FELDSPAR.... 24.0
Frit 3249....... 20.0
Zinc Oxide......  2.0

or using Frit F69:

G1215O
EPK............. 14.0
SILICA.......... 25.0
WOLLASTONITE.... 16.0
F-4 SPAR........ 25.0
FRIT F69........ 20.0
Zinc Oxide......  2.0

Because of high MgO this glaze is hostile to the development of color in Chrome Tin pink stains. It thus makes sense to add 1-2 percent zinc to make it work better with oranges and browns. We have tried it with about 20 other Mason stain and it works well with all of them (including manganese alumina pink, lavender and yellows).

Color Problems

This formula does not work well with chrome tin pink colors. To make it work you must reduce the boron and increase the calcia until it does. For the brightest pinks you may have to tolerate an increase in thermal expansion and another clay body higher in silica may have to be used to prevent crazing.

Since Frit 3134 and equivalents soften very early it is important that as much of the gases of decomposition from the body are expelled as possible before the frit melts and flows too much. If you get pinholing or blistering, try slowing the firing down during the phase just before the frit begins to melt. In this case it is around 1300-1400F.

Are You Ready for a Matte Version of this Recipe?

Yes, we have a very good matte version that can be tuned to fire with a surface very close to the classic dolomite magnesia mattes of cone 10 reduction (see links).

Conclusion

If you feel this was all too complicated, take heart. All of us are in over our heads, the ceramic process is exceptionally complex and none of us are really in control. So we need tools to better understand it. The oxide viewpoint provides a solid framework within which we can grow and learn together, even criticize each other. Yes this framework gives you the opportunity to explain how I am totally wrong about something in the above analysis. Then this base glaze would get even better. So let's at least start the new journey.

Links to Other Items

• G1214M Cone 5-7 20x5 Glossy Base Glaze
• Bringing Out the Big Guns in Craze Control: MgO (G1215U)
• Low Budget Testing of the Raw and Fired Properties of a Glaze
• G1947U/G2571A Cone 10/10R Base Matte/Glossy Glazes
• R-43
• R-3
• R-45
• R-46
• R-4
• R-37
• R-44

Authors

• Tony Hansen (Owner)

Much more information with complete interlinking to many related databases can be found by logging into the www.ceramicmaterials.info database Copyright 2003 http://digitalfire.com, All Rights Reserved Please support http://ceramicmaterials.info to improve this library

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