It’s been pretty cool to watch Ceramic Materials Workshop develop into what it’s become. Matt and Rose Katz are building an amazing thing, and their list of glaze disciples pushing the envelope of glaze magic has grown steadily. I’d planned to become an initiate myself and take the October class… but life on a remote island in Southeast Alaska has it’s own demands on my time, attention, and energy. That said, I just got this info from CMW in my email, and it looks great – certainly in line with what Ceramic Action is all about. I’d encourage you to check it out, you can not go wrong. You can even use the code CMWRULES and get the whole shebang for 75$ (Less than the cost of my average bar tab)!!!
This self guided workshop is a comprehensive, 54 part study into Mid-Temperature glazes. Also known as Cone 6 or Cone 5-6, these glazes are one of the most popular firing temperatures in ceramics. They can be beautiful, enticing, engaging and incredibly frustrating. In this workshop we will explore in depth, what we know to make the most out of Mid-Temperature glazes. We will take the mystery of materials, formulas and firings and translate them for real working artists.
I was beyond excited to work with my newest found material, a rhyolite from Topaz Mountain, in Juab Country, Utah. This time rather than choosing a handful of very large rock samples (to insure relative material consistency), I instead went to a wash and filled up a 5 gallon bucket with very fine material the size of course sand. My reasoning this time was that consistency is completely relative, and as long as I get materials from the same spots, it doesn’t matter – and I can grab material that has already been 99% processed for me. In the end I think this worked out, because I was able to run 5 gallons of sand through our ball mill with 2x 1 gal. ball mill jars in 10 batches. But I’m getting a bit ahead of myself, because I think it’s important to test fire a material before you go through the trouble of ball milling. So my new first step in dealing with materials (after identification of course) is to take a small chunk, put it in a small dish, and fire to cone 10 in reduction. Since this is my primary temperature range, that’s it, if there are chances I’ll also put similar samples into cone 6 oxidation as well as an oilspot firing schedule, which is about cone 12 oxidation. Here was the result at cone 10, in reduction:
Looks a lot like a fired chunk of granite or feldspar. Onwards with the milling!
Someone asked me about my process for ball milling, and here it is: Fill a 1 gal ball mill jar 1/3 with mixed sized media (approx 50% 1/4″ balls, 25% 1/2″ balls, 25% 1″ balls) then fill the jar with 1/2 gallon of water, then fill the the rest of the container up with material until it’s about 2/3 full.) If I had more containers I wouldn’t exceed filling the jar 1/2 way, but my circumstances are what they are, and I haven’t needed to change anything yet, such as it is.
In reduction, this rhyolite material was surprisingly similar to my ice crackle glaze. I think with very little modification (a small addition of clay, bone ash, and maybe a bit of frit) I’m nearly positive this will look and feel like a Kuan, ice crackle glaze.
Once I had all of my material milled, I let it sit overnight and then drained off the excess water, leaving me with a glaze slurry with an SPG of 1.58 (That’s 79g of material in a 50cc syringe). That’s only important if you want to know how much material you have per given volume. Since I was going to blend this with a basalt material that was also in solution, I needed this info. After taking the SPG of my basalt material, which happened to be 1.54, I did a simple line blend. On both sides are the materials by themselves, in the middle a 50/50, and on the left and right middle 25/75.
Pretty interesting results, I think. The big surprise was how sweet the 25% Basalt and 75% Rhyolite mix came out.
Finally, because I was looking for an oilspot/tenmoku type glaze with this research, I should also detail my firing schedule. Here’s my current Blaauw gas kiln firing schedule:
time_temp 00:00 5
time_temp 01:30 200
time_temp 07:00 1160
time_temp 01:30 1200
time_temp 01:00 1220
time_temp 02:00 1230
time_temp 01:15 1252
time_temp 00:08 1252
time_temp 00:30 1220
time_temp 01:30 1200
time_temp 02:00 1000
time_temp 02:00 800
time_temp 02:00 700
time_temp 02:00 500
time_temp 02:00 300
time_temp 02:00 50
time_temp 04:00 50
Blaauw kilns have the capability of firing in extremely oxidized conditions – blowing in somewhere to the tune of double the amount of air needed for complete combustion. The default, and maximum air value is 200. An neutral flame is around 100, and a smoky reduction is something like a 70.
Basically, this program fires up to cone 6 in about 9 hours, and then goes slowly up to 1252C, reduces for 8 minutes, and then goes back to oxidation, drops to 1220 over the course of 30 minutes, then drops to 1200 over the course of an hour and a half. I’m still very much tweaking this schedule, which works very well for some glazes, and not so much for others.
I got a lot of responses after my last post, where I made the claim that using zinc is hard on elements. I had some really interesting discussions with both John Britt and John Tilton on this subject, and I thought I’d share some of that info with you guys. Of particular interest was a tidbit from Britt where he mentioned that firing a zinc sample in oxidation leaves it unmelted (as the melting temp is 3500) while the zinc sample in reduction almost entirely goes away. Heres a fired sample from our glaze calc class, fired in cone 10 reduction. It was a thumbnail sized lump when it went in.
To make a long story short, the juries out on whether or not zinc directly reduces the life of kiln elements. What’s most likely is that it’s a combination of complex volatilizing compounds released during the firing coupled with high temperatures (especially if there are programmed holds). An often overlooked variable is water. It could very likely be that the extremely hard water at my current and previous studios is the culprit. If you’ve ever looked closely at old workhorse kilns, some of the deposits near the lids and spy ports are similar to calcite deposits. Its not hard to imagine Calcium and Flouride attacking the elements.
Another variable to consider is that the kilns that I’ve had access to for my entire career have been heavily used community kilns. At research universities there’s a huge range of clays and glazes that go through kilns. I’ve also heard that barium, cobalt, and copper are also hard on elements. Given all these extra variables, I suppose the only real way to quantify if and how much zinc affects elements would be to fire two brand new kilns side by side for a 100 or so firings and then compare. Let me know if you have two brand new kilns you need me to test. I’ll be happy to help you out!
On a related note, John Tilton had some great info on kiln elements. I hope he doesn’t mind me sharing the info – it seems very useful. From an email from Tilton:
The zinc thing is somewhat solved by using heavier elements. I have 11 gauge KA1s in my newest L&L, and after 104 firings they are still standing nicely. 12 gauge seems to give about 200 firings, and 13 gauge 80. Normal elements, the 16 gauge or so, give between 18 and 35 firings, not worth using. I also fire only one or two pots at a time so the total zinc load per firing is probably less than if you stacked tightly.
So it looks like 11 gauge might be the sweet spot, though they do require length to be resistant enough, and maybe that translates into coils of larger diameter.
Finally, I’ll leave you with the Material description from Digital Fire. I highly recommend consulting digital fire for material descriptions, and if you have the means, purchase access to the software. So worth it. From Digitalfire.com
Enter the formula and formula weight directly into the Insight MDT dialog (since it records materials as formulas).
Enter the analysis into an Insight recipe and enter the LOI using Override Calculated LOI (in the Calc menu). It will calculate the formula.
Zinc oxide is a fluffy white to yellow white powder having a very fine physical particle size (99.9% should pass a 325 mesh screen). It is made using one of two processes that produce different densities. The French process vaporizes and oxidizes zinc metal, the American process smelts a coal/zinc sulfide mix and oxidizes the zinc fumes.
Ceramic grades are calcined, they have a larger particle size and much lower surface area (e.g. 3 square meters per gram vs. less than 1; however 99.9% still passes 325 mesh). Calcined grades are said to produce less glaze surface defect problems (although many ceramists have used the raw grades for many years without serious issues). You can calcine zinc on your own in a bisque kiln, fire it at around 815C. Calcined zinc tends to rehydrate from atmospheric water (and get lumpy in the process, calcining a mix of zinc and kaolin produces a more workable powder). Alot of zinc is used in crystalline glazes (typically 25%), because these have no clay content, they bring out the best and worst of both the calcined and raw materials. The raw zinc suspends glazes much better (the calcined settles out much more). The raw zinc takes more water, but since the glaze can thin out over time it is better to add less than needed at mixing time and mix thoroughly. The raw zinc screens better (although it can be a challenge to get either slurry through an 80 mesh screen).
Zinc oxide is soluble in strong alkalies and acids.
It can be an active flux in smaller amounts. It generally promotes crystalline effects and matteness/softness in greater amounts. If too much is used the glaze surface can become dry and the heavily crystalline surface can present problems with cutlery marking. Other surface defects like pitting, pinholing, blistering and crawling can also occur (because its fine particle size contributes to glaze shrinkage during drying and it pulls the glaze together during fusion).
Zinc oxide is thermally stable on its own to high temperatures, however in glazes it readily dissolves and acts as a flux. Zinc oxide sublimes at 1800C but it reduces to Zn metal in reduction firing and then boils at around 900C (either causing glaze defects or volatilizing into the atmosphere; note that electric kilns with poor ventilation can have local reduction).
While it might seem that zinc would not be useful in reduction glazes, when zincless and zinc containing glazes are compared it is often clear that there is an effect (e.g. earlier melting). Thus some zinc has either remained or it has acted as a catalyst.
The use of zinc in standard glazes is limited by its price, its hostility to the development of certain colors and its tendency to make glazes more leachable in acids (although zinc itself is not considered a hazardous substance).
Zinc oxide is used in glass, frits, enamels and ferrites. Zinc oxide is also used in large quantities in the rubber and paint industries; in insulated wire, lubricants, and advanced ceramics. Credit: Tony Hansen
Thanks to Tony, and John Tilton. Also a special thanks to John Britt for the feedback on my last post; the clarification on Feldspar (I was mistaking K200 for Minspar 200 – ignorant to the fact that K200 has been out of production for 20 years!) I’ve since edited my post on Cone 6 with this info.
I get asked a lot about this recipe, and for good reason. It’s pretty indistinguishable from the best cone 10 recipes out there. For those purists out there, I’m referring to Pinnell Clear, Deller Chun, Cushing’s LungChun, and any number of Robert Tichane’s recipes from his book Celadon Blues. In any event, I often point people to an older post, but in the years since I mistakenly transcribed a recipe wrong and happened on the winning formula, I’ve learned quite a bit working with this glaze; Things like it working reasonably well in soda and atmospheric kilns, looking very nice from a range of cone 5 to cone 12, and readily taking most mason stains.
Fiske 6/10 Clear Base:
F4 (Or MinSpar) Feldspar 34.9
Zinc Oxide 11
OM4 Ball Clay 13.8
(Pictured: Add 1.75% Robin’s Egg Blue Mason Stain)
Notes on materials, mixing, and application:
Feldspar:Since F4 is no longer widely available, Minspar 200 will work. Custer works as well, but the bubble matrix that really gives this glaze it’s character is different with custer, g200 (now g200hp), or nepheline syenite. Experiment first, because milage may vary. Of the ingredients, this is probably the 3rd most important.
Fluxes: Whiting and Zinc. This glaze is not kind to kiln elements. (See my post on zinc for clarification!) It’s my opinion that the relatively high % of zinc is caustic to electric kiln elements. If you must, ventilate the kiln, but expect a short life on the elements. Sometime in the near future I’ll be eliminating zinc and trying to use a frit to solve this, but until then I can only recommend firing in a gas kiln. It’s the cost of firing.
Clay: Probably the most important element of this recipe. When I was testing for cone 6 glazes, I made a mistake transcribing to a batch recipe. The result was that I had doubled the clay. After the firing I went back over the notes and realized why the glaze looked the way it did. One of the side effects of the higher clay content is that application is sometimes difficult. The higher % of clay makes thick applications crawl. To get around this I calcine 10 of the 15%.
Silica: I use 200mesh sil-co-sil. I’ve tried 325 mesh, but it didn’t look right.
Colors/Mason Stains: I use Robin’s Egg Blue, Bermuda Green, and Canary yellow. Most colors I’ve tested, and usually 1.5-3% is pretty nice, but some take as much as 5-10%. I haven’t had much luck with purples, Pinks, and oranges, (they don’t play nice with the zinc) but honestly I’m largely done tweaking this one and haven’t tried in earnest to figure out those other colors. Metallic oxides will also work, cobalt at like .3% for a not overpowered blue color.
Application: Can’t stress this enough. It’s gotta be thick. I tell people to glaze “Thicker than you think thick is, and then just a little thicker.” I’ve taken to adding just a touch of deflocculent to the glaze batch so that it needs less water to become liquid. I then add a bit of Epsom salt to thicken the batch up. Again, this is to taste. Dipping is absolutely the way to go with this one, but I’ve gotten accustomed to spraying it. Usually takes about 15 minutes to spray glaze something appropriately.
Firing: As I mentioned earlier, it’s got a pretty wide range. It will be fully melted, albeit slightly pin holed at cone 5. Ideally, I like to go to a perfect 6, but taking it to 7 or programming a hold in the schedule makes for some nice movement that suits carving and texture very well. Most of my work is completely smooth, so I prefer it to stay thick and not run down too much. It takes some getting used to, but when you do, it behaves very predictably. It can also go into reduction, but the colors change quite a bit. Less change with Bermuda Green, but quite a bit with the Robin Egg Blue. Its been fired every which way, and needs to be tested before full comittment.