Potters & Sculptors - Making Rock from Mud
First off, I'm really glad I found this place! Looks like a great resource and exactly what I need as I've just bought a kiln, and am looking to fire to ^6.
I've never fired a pot myself before, the few things I've made went to my wife's evening class to get fired. I bought her a wheel for her 30th, and started playing with it myself. I really enjoyed throwing but was frustrated I couldn't fire anything - my wife stopped going to classes.
So late one night I was browsing Ebay, with a glass of wine in hand. I had been looking for a kiln for a while and found one counting down, on a whim I bid and to my surprise won the auction.
It's a 23inchx18inch Olympic S1823, from my minimal knowledge the bricks and elements look in decent condition, though there is a rather large chunk missing from the brick on the top layer. From what I've read, as long as the elements are secure it should be ok though...
I didn't quite realise how big it was going to be but somehow I managed to lift the thing out of my car and into the garage on my own.
After ages trying to track down an electrician who would run a suitable supply to it, the kiln is finally ready to fire. I turned it on briefly when it was installed, and it gets warm, but didn't have time to do a proper firing.
So fingers crossed, all the elements work and it gets to temp etc. I found this firing schedule on lakesidepottery.com I was wondering if it looks ok? I thought bisques normally take around 5 hours, but the one here is more like 9 hours...but I guess it depends on your kiln and things.
This schedule uses the same type of kiln as mine, with three dials to control temperature
I plan to bisque to ^07 and glaze at ^6
Ok, any thoughts are welcomed!
APPENDIX B - Manual kiln firing example - (for a bisque firing):
Place a cone 08 into the sitter, and prop the lid open with a 2-3" piece of soft brick. (using a hard post gouges the kiln top surface and lid).
0.00: Put the bottom elements on LOW. Leave the peepholes in (this prevents a chimney effect from cooling the front of the kiln.) Leave the bottom on LOW for 1 hour.
1:00 Put the middle elements on LOW. Now the bottom and middle are both on LOW. Leave the kiln like this for one hour
2:00 hold a small mirror near the front of the open lid. If condensation appears, let the kiln go for another hour. If it does not, drop the lid.
3:00 turn the top element to LOW
4:00 turn all switches 1/3 of the way on (8:00 if you liken it to a clock face)
5:00 turn all the switches to 1/2 on (6:00 on a clock face)
6:00 turn all switches to 3/4 on (3:00 on a clock face)
7:00 turn all switches to HIGH. Dull red heat should begin to appear within the hour. When it does, set the timer on the Kiln Sitter/Timer to 2 hours. The kiln should shut off within that two hour period, most likely before the timer runs out.
This is just an example of a typical firing schedule and may need adjusting at different places depending on your own conditions (e.g., clay and ware types).
So I fired my kiln today! It worked! I'm very pleased the first time I used a kiln it didn't go up in flames or anything!
I followed the schedule above, it did seem to take forever for moisture to go, I held a mirror above top hole and it took 4 hours before I could get above the 'low' stage.
In the end it took about 9 hours to fire to ^07. The garage where the kiln has been sitting is a bit damp. Some of the pots were stored in there too so weren't totally bone dry I think.
I wondered if the kiln bricks will absorb moisture?
Still not sure about my schedule for glaze firing - hopefully if I bring the pots indoors to dry after glazing it wont take too long to fire.
Tom regarding glazing and glaze firing that is very different from a bisque. When you get ready to apply glaze wear gloves and rinse the bisque piece to remove dust before you glaze. Apply your wax resist to the bottom and then glaze. Try not to handle the wares after you have glazed and place them in the kiln as soon as you can. Dust is not friendly in glazing and oils on your hands can cause imperfections. Also fire your glaze fire SLOWLY (12 hours for cone 6) then when it finishes resist the temptation to open it while it is still warm. Good luck, can't wait to see your pieces.
Get to know your kiln.
Our kiln takes 9.0 hours to fire to a Cone 04 bisque on SLOW and 5.5 hours at MEDIUM-SLOW speed. Keep in mind our kiln is enormously over-powered using 220 volt to provide 10 kWatts of heat per cubic foot. Even with three kiln elements broken, our kiln can maintain its firing schedule. With most kilns providing less power per cubic foot, a longer firing time can indicate a kiln element is out. To test the elements we just stick a small strip of paper on each element and fire the kiln at FAST for several minutes. Any unburned paper identifies a broken heating element.
Kiln bricks, greenware and glazed bisque all absorb moisture. It's smart to do a preheat to eliminate this moisture, but it shouldn't otherwise make much difference to the firing time.
We have a computerized kiln controller so can easily add a 200 F preheat to all of our kiln firings. In the Winter the preheat needs to be longer than during the Summer, like today with the temperature at 101 F.
Hi guys, thanks for your replies, I took all your advice on board.
So we finally did our first glaze firing - results are mixed but overall I'm happy we made it to ^6 and some of the glazes worked, I just need to work on making prettier looking pots now!
Vase with glaze chip, I think it may have been while cooling. I opened the kiln 9 hours after it hit max temp, then closed it because I realised it was too warm. I then waited about 8 hours to re-open. Would thermal shock do this, or maybe just dodgy glazing?
A lidded jar
My Wife's lovely flower bowl
Witness cones from shelf near bottom - 7,6,5 - should have ordered them the other way I guess, seems to be a decent ^6 firing though? Another newbie error I made - Set the cones across from spy hole, looked to make sure I could see them. Then stupidly put a prop right in front of spyhole to obscure the view, and didn't realise until I looked in after about 4 hours of firing. Luckily the kiln sitter seemed to work well this time.
My wife thinks this glaze was too thick, that's why it has done this...? Is it called creeping? I had actually glazed this before and washed it off because I wasn't happy with it.
But its the same glaze as this bowl, but maybe I added more water after the mug. It looks like i need to sieve it anyway, i don't currently have sieves fine enough so need to get some before next firing and put all the glazes through them.
Your first experience with shivering/dunting and crawling.
Crawling is where the molten glaze withdraws into 'islands' leaving bare clay patches. The edges of the islands are thickened and smoothly rounded. In moderate cases there are only a few bare patches of clay, in severe cases the glaze forms beads on the clay surface and drips off onto the shelf. This can occur when the surface of the bisque is not clean or is not completely dry.
Glazes high in magnesium have so much surface tension that they crawl by design. Crawling is common in once-fire ware as off-gassing and steam from the clay body displace the glaze.
Sometimes a piece of glaze will crack off, normally near a rim or at edges. Some clay may be attached to the glaze piece that cracks off. This occurs because stress has built up between the clay and glaze that can't be absorbed. It is often caused by over-sponging which takes away the fine clay particles and leaves behind the groggier clay particles which are not elastic enough to absorb the stress. Additional shivering can also occur over time if the glaze has a lower COE (coefficient of expansion) than the clay body.
Crashing a Kiln
Opening a hot kiln puts stress on the kiln and elements. Stick a piece of paper into the peephole before opening a kiln without a thermocouple. But a temperature above the alleged Fahrenheit 451 can still be too hot to open a kiln if the ware has been high-fired and has developed cristabolite, which shrinks 3% at 428 F / 220 C.
So if I ever "crash a kiln" by opening the lid when the kiln is too hot, I want to re-close the lid at 450 F and not re-open it until the temperature is below 400 F.
Quartz inversion at 1,063 F / 573 C, where quartz shrinks roughly 2%. So the kiln should also be cooled slowly during the temperature range. But not many would be tempted to open the kiln lid this hot.
Wow, what a great fun firing! I remember my first firings in a manual kiln, going at it solo also. Had some of my best glazes come out of my manual kiln, and you will too!!! We have all had some of the same issues you had, and still do, crawling glaze, yep, saw that when I just opened my kiln. Too thick of glaze on the pot, starts to dry and crack on the pot, like an desert island of dry cracked mud. This time I was actually after that effect. Ceramics is so fun, and frustrating at the same time, sounds like you are truly hooked, like the rest of us! You will learn so much from every firing!!!!