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.

Long time no blog

It's been a while since I've blogged because I've been fishing a lot and not making pots so much. I do however have the studio in place pretty much. At least enough that I can wedge clay and make pots. So I'm doing that and awaiting the cash flow to complete the kiln project. I'm only short a few hundred bucks, but it's amazing how much a few hundred bucks is when you don't have it to spare.

The great thing is I've been able to get my feet wet with castable cement in the building of a backyard cook center for my mom. We've built a catenary arch kiln style oven with a small arched firebox in the bottom and a level cook top in the oven chamber. I'm pretty excited to finish the piece off this weekend. I'm going over there to build the final frames for the big catenary arch which is going to be the top, sides and back of the oven. 

To build this thing we've used lincoln 60 fireclay, pine shavings, fine and course mulcoa grog, and Ciment Fondu which is a 40% alumina refractory cement. This was used in a 2,2,2,1 parts formula. 

2 Fireclay

2 Pine Shavings

2 Grog

1 Ciment Fondu

We had to substitute some sand to complete the cook top because we were running out of aggregates at the time. We also had to substitute some Portland Type II cement into the cook top because we ran out of Ciment Fondu. So the top of the firebox itself has about 2 inches of good refractory cement, and the top of the firebox layer or cooktop has about 2 inches of Portland Cement. For this application it should be just fine, although I did have the idea that I would burn this thing to red/orange one time to cure it and I don't see that happening with Portland Cement. We'll see how it goes, but I certainly don't want to push the cook top the the point that it fails.

The tall arch we're about to build is going to be a 2" layer of medium density castable just like the firebox pieces were, followed by a 3 inch layer of light castable made with perlite. In the big arch we're going to put holes which will later hold spring mounted bolts for a flange which will hold the doors. There will be two doors on the oven one small one down below for the firebox and one up top for the oven which will be quite large.

Keep it Moving

It's been a period of constant transition for the last year. I've been unable to set up shop for very long because we keep having to up and move. I'd planned this time to stay here for at least a year, but here we are again in a position where we need to move. The current living arrangement has failed to work as intended and I have to give up my two car garage, my huge office and my new garden space that's full of vegetable plants. I'm really bummed out!

So we're looking at places somewhere outside Vancouver. A couple look promising, but no concrete plans as of yet. One little place on an acre in Orchards is particularly interesting, but it has no garage or basement. I could build a cheap out building made of Plastic and cattle panel for a greenhouse and a shop.

The good news is, I have done the research and can now build a large castable kiln for a few hundred bucks. I'll be building the arch frames and ramming my mom's oven by the end of the month.

The bad news is the move is going to cost me all my student loan money and I'll likely lose all my vegetable crops when I go.