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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. |
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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:
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.
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.
Cone 6 Limits: Green & Cooper |
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| CaO | 0-0.55 |
| ZnO | 0-0.3 |
| MgO | 0-0.325 |
| KNaO | 0-0.375 |
| B2O3 | 0-.35 |
| Al2O3 | 0.275-0.65 |
| SiO2 | 2.4-4.7 |
| Boron unified with other fluxes | |
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.
| B2O3 | Boron is a low melting glass, it is like silica on steroids, it is a flux
and a glass. It is possible to make a cone 6 glaze without boron but it won't flow nearly
as well and it will tend craze. Lots of cone 6 glazes have way too much boron, some
employing Gerstley Borate to supply as much B2O3 as all the other fluxes combined. Not
good, it should be one third of that. Why? Durability, solubility and reversal of the
thermal expansion influence. Like steroids, use it when appropriate and in the right
amounts only! I get very good melt fluidity when B2O3 comprises about 25% of the fluxes.
However chrome-tin colors tend to start failing if the boron is more than one third of the
CaO amount, more on this later.
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| CaO | CaO is a strong flux at cone 10 but not as much so at cone 6. But having
some B2O3 around can really encourage CaO to flow better. Likewise the presence of sodium
and potassium can turn CaO into a much more active flux (melter). We need CaO as high as
possible to make the glaze work with chrome-tin colors, so I would like to approach the
maximum recommended in the limit formula. However CaO can really tend to form either
calcium-silicate or calcium-borate crystals on cooling if oversupplied. This is a bit of a
dilemma. To get around 0.55 formula units of CaO (of a total of 1.0) we need to top up the CaO already supplied by the frit. This will take 10% of my favorite calcium mineral, Wollastonite. It is much better quality than whiting, contains finely dispersed SiO2, and it does not form gases of decomposition. |
| Na2O/K2O | As noted above, these oxides are a good choice to round out the fluxes to
a total of 1 (unity formulas are adjusted so the fluxes total 1). Na2O and K2O behave in a
similar manner and melt well at cone 6 to encourage bright colors. I chose to use a clean
and consistent feldspar named Kona F-4, 15% is needed. The feldspar also brings some
alumina and silica with it. An interesting part of supplying this on the computer is that the feldspar also sources CaO so you have to go back and adjust the Wollastonite again to restore CaO to 0.6. |
| Al2O3 | Al2O3 gives the glaze melt body, stiffens it to keep it from running off
the ware, even over a range of temperatures. The stiff melt also prevents the growth of
crystals that disrupt smooth glaze surfaces and cloud transparents, we want that in a base
glaze. Alumina lowers glaze thermal expansion very significantly and is a key to glaze
hardness and resistance to leaching. Alumina and boron melt together very well. Like SiO2,
you want as much alumina as a glaze will take and still melt well (you need good reason to
under supply Al2O3 i.e. non-functional crystalline glazes). High Al2O3 usually means a
matte glaze, but not if there is significant B2O3 and SiO2 glass formers around. Boron glazes can take a lot of Al2O3, the above chart claims 0.65 parts (compared to around 0.4 for non-boron glazes). Tests I have done show that with the above amount of boron a glaze in this system will comfortably take 0.35 alumina, melt very well and prevent crystal growth. By 'melts well' I mean it has fired hardness, minimal bubble population, smooth hard surface, and good melt flow. Kaolin supplies Al2O3 and it produces a glaze slurry that gels, applies evenly to ware, and dries hard. This is a match made in heaven, the more kaolin a we can employ the better. What kind of kaolin? I prefer EP Kaolin because it is clean, plastic, and flocculates and gels the glaze so it applies without drips, even on dense bisque ware. |
| SiO2 | Like alumina, silica has many beneficial effects on the fired glass. It reduces thermal expansion and increases glaze hardness; the more a glaze can tolerate the better, the limit formula above recommends up to 4.7. It is true that too much will compromise matteness or raise the melting temperature, but it is surprising how many glazes will take plenty of silica without any trouble. For example, I ran many tests with around 2.5 parts SiO2 thinking that it would not melt well with more. Then I added a full 10% silica (the equivalent for going from 2.5 formula to 3.0) and it worked perfectly, completely solving a crazing problem on a low silica porcelain I use. It is possible this recipe will tolerate even more. |
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*
FLINT 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 in general visit http://digitalfire.com/material, to research oxides visit http://digitalfire.com/oxide.
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 that I have documented at http://digitalfire.com/education/glaze/cone6.htm. I am promoting a way of thinking. Take this recipe, understand it, adjust it, make it work for you. For example:
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.
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. There are simple and inexpensive ways to check each of these and you can find them summarized on the web page at http://digitalfire.com/education/glaze/testing.htm.
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. One place to go is the Internet. Visit the web page at http://digitalfire.com/news/links.htm for a good start.
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.
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
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.
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. Visit the page at http://digitalfire.com/education/glaze/g1214z.htm.
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.
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