Glaze Research

Digital Fire has a lot of free links that hold a wealth of information. You can find chemical data on nearly all individual materials, as well as a number of formulated glaze recipes. What I've been working on the last week is trying to figure out proper formulations for durable matte glazes in both Cone 1 and Cone 6. I have found a myriad of recipes for high gerstley borate matte glazes. Which I know from first hand experience from testing in college that these glazes are not durable and they leach. So I'm trying to come up with a formula made from the materials I have on hand that make a matte glaze. The good news is I've found shiny base glazes for both the cone 1 and the cone 6. I've also found some proven cone 1 and cone 6 macro crystalline glazes. I have a lot of testing to do now that I've done some research. However for anyone who might be interested in these types of glazes here are some links to digital fire that might help.

http://digitalfire.us/4sight/education/g1214w_cone_6_transparent_base_glaze_121.html

http://digitalfire.us/4sight/education/g1214z_cone_6_matte_base_glaze_122.html

http://digitalfire.us/4sight/education/formulating_ash_and_native-material_glazes_188.html

All three of these links are great. The last one is of particular interest because I have an ash deposit that contains a known material cataloged by a geologist with the USGS.

Good in Bad

You know how they say things happen for a reason. Well this recent firing made me read up on a lot of stuff I'd already learned but had maybe not remembered lately. It also made me do a bit more digging to find some things I did not know.

For some time now I've been interested in doing crystalline glazing in my studio. In fact it was Wally Schwab who first introduced me to crystalline glazes when I studied under him at PCC Rock Creek. I'd also done quite a lot of research into microcrystalline glazes at MHCC with Don Sprague. Since then some advances have been made by other potters to fire these glazes at cone 6. When I looked at these before I'd found a cone 1 crystalline glaze as well, but had lost the recipe. Well I found it yesterday, so I'll be tinkering with that one soon thanks to Charlie Cummings post on Clay Art Thread. I also did some research into the firing schedules and found out that the crystallization is happening upon cooling, but a very slow cooling where the kiln is held at the 1830 - 1900 degree mark for four hours. What's interesting is this is when the iron in cone 5 iron reds is held to facilitate crystallization as well.

As you may know there is a huge difference between Orton cones. The large ones drop much faster than the small ones, in fact a large cone 6 drops at 2228 where a small doesn't drop until 2291. Sixty three degrees doesn't seem like a lot when you're talking about thousands of degrees, but it's enough that it could make a glaze go from stable to runny. It's not just the temperature but also the time it takes to get to the higher temperature.

Charlie Cummings
Macrocrystalline Cone 1
Frit 3110 - 43
Zinc Oxide - 23
Silica - 16
Lithium - 9
Laguna Borate - 9

Firing Up, Firing Down

After doing a lot of reading over the last several days. I think I've found the answer to the glaze firing problem I had recently. The potters who fire reds fire up and then ramp back down slowly. Some shut the kiln down and then manually hold the setter back on for 20 minutes in the 1800 - 1900 range to allow the crystallization of the iron molecules that reflect the color red. This is where Crystalline glazes also ramp down to and hold for a number of hours. Some potters however who were doing reds actually fired back down slowly to achieve the red coloration. This is also used to more slowly cool a kiln to allow a glaze to stop bubbling and heal over. In my case the cone malfunctioned and I did not have witness cones inside (big mistake) to tell me when I'd reached peak temperature. I've decided to do my glaze firings based on witness cones instead of relying on the setter to shut it down. This way I can fire back down slowly cooling the glaze.

Misfire

After some consideration I'm fairly certain that my trouble with this firing was a cone malfunction combined with a quick cooling. I did a leak test on some cone 10 pots I put in the firing and they are vitrified. Some parts were unglazed and did not leak. The cone 6 glazes I used were supposed to be matte and completely melted and ran down the pots. The kiln took a very long time to reach top temperature, like 4 - 6 hours longer than I expected. I'm fairly certain that the kiln actually hit cone 8 - 10 range. I'm going to purchase a packet of cone 5 and a new box of cone 6 and try these glazes again.

Lack of success

Well my first cone 6 firing in my new location was a failure. The kiln cooled too quickly trapping bubbles in the glaze on all the pots. The matte transparent also came out a shiny and clear which leads me to believe that the long time the kiln took to reach temperature made the glaze over mature. It became fluid and started to run down the pot as if it were not a matte glaze at all. Quite disappointed in this one, however I did get some great color response in both the purple and the blue. As Churchill said "You can always count on American's to do the right thing, after they've tried everything else." Looks like I need to wrap my kiln is fiber or something.

Cone 6

Wow my small electric kiln takes a long time to reach cone 6. I'd been doing cone 1 for a while now which is about the top end of lowfire. Cone 1 makes vitreous durable pots out of low fire white clay. However the clay has a sticky feel and is not as nice to throw as cone 6 porcelain or stoneware. Besides I've got some great cone 6 recipes to go with some really great cone 6 clays that I enjoy working with. I'll continue to do cone 1 because it's fun and cheap.

I have a bunch of cone 6 porcelain that I really didn't like so I wedged in some mulcoa 47 grog 35 mesh, which stopped all the finicky problems that high spar content clays have. With about 10% added grog to the wet weight, the porcelain takes on a whole new personality. It retains it's plasticity, and gains structural integrity, and fast drying capabilities. This clay went from S cracking while drying unless you kept it under plastic for days and flipped it on the rim while it was still quite wet, to being able to throw plates and leave them until the rim and inside is leather hard, flipping and trimming the majority of the foot while wet, then coming back and finishing off the last bit once it's leather hard. Never being covered in plastic and drying in the kiln room at about 85 degrees of dry hot air in the room. I love porcelain, but the forgiving nature of a clay with grog is just so much more satisfying. With a full schedule of classes I need a forgiving clay that will wait if I want it to, but can dry quickly if I want it to as well.

I've also been making some "Patrick Horsley pots" which is really my take on his techniques. I've really been enjoying the method which has opened a whole new design element to my work that was not possible before I took his workshop. I've taken to making these figurative jars that are flattened to an oval and have a wider shoulders and body with narrower bottom half and narrow neck. I find them fun to make as well as aesthetically pleasing. The porcelain with grog has been an effective clay in making these pots.

Research

I'm a big fan of ceramics books. The one drawback of my book library is that a lot of it is dated material in classic ceramic books. One such thing I found was Jack Troy's book Salt Glazed Ceramics. The book covers how to make salt glazed ware, and salt kilns. In his book he used Portland Cement because the newer alumina refractory cements weren't available when he wrote the book. Toward the end he discusses the ongoing research in castables which he discusses adding alumina to the cement in ratios. Well I did a lot of reading and found many articles talking about castables and several covered this in detail claiming that adding alumina oxide or alumina hydrate to a cement would not work.

I did a lot of reading on castables and refractories. I read research papers books, blogs, discussion boards, websites, technical data sheets etc. All of this research led me to a basic formula for a medium to light castable cement which was mentioned in my last blog consisting of, 2 parts fire clay, 2 parts grog, 2 parts saw dust, 1 part cement. For this recipe I chose to use Ciment Fondu because of the fairly reasonable price of $44.00/bag-94lbs.

We used this product in my mom's backyard oven. I have some concerns about the oven because the castable will shrink considerably and my parents decor design has a flaw. They wanted to use bricks on the outside and we'd already decided to use a castable cement. Well the cement is going to shrink some when I fire the oven off the first time, and the bricks will not shrink. This means that the interior will shrink away from the bricks and there will be a gap. This would not be a problem except we used a sprung arch design in the top of the oven for maximization of space. As it shrinks it can become unsupported and crack, or even fail. I'm going to strap the thing together with steel support braces prior to firing, but who knows if it will work or not... The one thing I could not find (and did not test because of time constraints) is the shrinkage of this castable upon firing. I guess I should hurry up and fire off the blocks I've made.