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View Full Version : Newbie: please help w/firing advice



jgourlay
02-01-2007, 10:22 PM
Greetings all, and happy new year!

I am a total newbie to this and I've come to it as a good way to spend time with my 6 year old doing something other than television.

Thus far I've made 3 pots, with 1 firing successfully, and the other exploding owing probably to excess moisture. To fire, I'm using a bbq grill with internal dimensions of 12" deep x 23" wide and 6" tall. To date, I've used a mixture of mostly grilling charcoal with some coke thrown in. I can hook up propane, but not at a satisfactory rate of flow, although it does make a good adjunct to the charcoal.

The clay is a "Raku C" with "kaynite". The glazes I've used so far are: Envision foodsafe (Blue, yellow, and clear) as well as a supersaturated solution of salt (NaCl).

I am having a problem, and would appreciate your suggestions. What I am trying to achieve are (primarily) a "foodgrade" finish and (secondarily) pretty colors.

What I have found so far is as follows:

1. If I do ONE firing the blue comes out okay, the yellow and clear come out grey, and all three look like the cratered moon. The salt comes out a nice mild red, but does not seal the porosity and hold water (no surprise).

2. if I do TWO firings (first without glaze, second with), the cratering disappears, but the glazes seem to disappear as well except the blue. The doesn't glaze, but appears to soak in. I didn't try water in that one, but the surface texture looks like the non-water holding salt from above. The yellow and clear are just awol. The blue is there, but turns a kind of dull grey with some bluish tinge.

Note that so far I am firing the pots in the middle of a big heaping pile of fuel, with both the coke and charcoal piled directly on top of the work. I lay down a layer of fuel maybe 2" deep, then the work, then pile up charcoal into a cone shaped pile until it just begins to spile over the edge.

Here is what I believe I can control at this point:

1. Heat: more charcoal or less and charcoal closer or farther from the work. This is going to be a pretty coarse adjustment.
2. Heat and Airflow: lid up, or lid down. Lid down, less heat, less air, longer heating period. Lid up, lots more heat, much faster ramp, airswirls causing much less thermal uniformity, faster burn (less time in fire). I can, of course, keep dumping on charcoal but pretty quickly ash plugs up the holes on teh bottom at which point only the top surface of fuel has the high heat.
3. Number of firings.
4. Thickness of glaze painted on.
5. Choice of glaze (suggestions?)

I know I am reaching vitrification temperature as the pottery doesn't melt in boiling water--the water just soaks in. Also, as a reference, pennies placed in the pot will burn away leaving no zinc core and very little of the copper shell.

Any suggestions?

tbobker
06-02-2007, 01:27 AM
What kiln are you using?

The cratering is probably happening because you have not fired it for long enough or soaked it (left at top temp for a period of time)
The reason it didnt when you did one firing first is because all the moisture was dried out and the glaze stuck to your pot better.

If you do a once glaze firing always do it really slow until 200 - 300 degrees it gets all the moisture out of it first. Make sure you soak your pots (as mentioned above) .

Typically you cant really get functional and pretty.....annoying! Earthernware tends to be brighter and more porous. You mentioned salt - are you doing a salt firing?

Remember with once glaze pots they need to really dry out before putting in the kiln otherwise they will explode, you also need to apply more layers of glaze becuase the moisture will just soak straight in.

If you are still want to mess around as you are i suggest you dont pile charcoal on top of your work, it is seriously uneven heat.
Try building a tower out of bricks with your grill half way up and stoke the charcoal under the grill you can use a congreate slab or something to put on top of the tower, not fully because you will starve the fire, but you can create some good effects with reduction (look up reduction firing) this is a really amazing technique especially with copper!!!! Copper red...mmmmm,....

Remeber the heat needs to be as even as possible, and as slow as possible toward 100-200 degrees, boiling point (explosions, moisture, gas, no escape) take at least 100 degrees an hour to 300 then speed up. To do a firing how you are where heat is probably really escaping will take a good 5-6 hours to 500-600 degrees low raku firing.

Make it easy for yourself, get some ceramic fibre and a oil drum...look up hand made kilns. THey are seriously fun especially if you have gas hooked up. Raku copper..my favourite.

Good luck!

Enchanted Ceramics
02-09-2007, 09:56 AM
If I were you I would spend a morning in the library. Get some really good books and make loads of notes. There is so much to remember. Write it down.
Good luck