|Monthly Tech-Tip |
Every gas kiln is different so this schedule may not work for you if you kiln does not cool at the same rather or maintain the same atmosphere. Our kiln is brick construction, downdraft, four natural gas burners, about 30 cubic feet. It fires evenly top-to-bottom and front-to-back. We have had and used others but none of them equaled the quality of firing this can produce. It was built by Luke Lindoe.
It can rise while reducing so we do not do a separate body and glaze reduction (with an oxidation climb between). Reduction is consistent throughout the kiln, enough to produce a clear bluish flame at the peepholes (with some yellow at the damper). We soak for half an hour in oxidation at the end. To prevent temperature rise during this phase we turn the gas pressure to almost zero (allowing it to drop a little). On shutdown we seal it up but temperature still free falls (although slower than a fibre kiln would). We open it at about 200F two days later.
|1||10°C/hr to 120C||18°F/hr to 248F||0||12:23||Heat up overnight on pilots|
|2||50°C/hr to 550C||90°F/hr to 1022F||0||20:59|
|3||100°C/hr to 980C||180°F/hr to 1796F||0||25:17|
|4||50°C/hr to 1300C||90°F/hr to 2372F||0||31:41||Start light reduction beginning of this step|
|5||0°C/hr to 1280C||0°F/hr to 2336F||30min||31:41||Oxidation this step|
|6||0°C/hr to 30C||0°F/hr to 86F||31:41|
I was consistently getting the cone on the left when using a custom-programmed firing schedule to 2204F (for cone 6 with ten minute hold). However Orton recommends that the tip of the self supporting cone should be even with the top of the base (they consider the indicating part of the cone to be the part above the base). So I adjusted the program to finish at 2200F and got the cone on the right. But note: This applies to that kiln at that point in time (with that pyrometer and that firing schedule). Our other test kiln bends the cone to 5 o'clock at 2195F. Since kiln controllers fire cone 6 at 2230 (for the built-in one-button firings) your kiln is almost certainly over firing!
It is old, but nothing we have ever used fires as evenly and reliably as this downdraft kiln. It was built by Luke Lindoe in the 1960s. I have used it through my entire life as a potter since the early 1970s. Two burners at 2 inches oil-column will take it to 1000F fairly quickly, but it takes 4 burners at 4 inches and 20 hours to get it to cone 10. I can judge the back pressure and degree of reduction by the length and color of the flames at the peepholes and color of the flame passing the damper at the back. Changing the damper position by an eighth of an inch during reduction is enough to discern a change in the flame.
Courtesy of Bailey Kilns.
GR10-A - Pure Ravenscrag Slip
Ravenscrag all by itself makes a great cone 10 reduction semi-gloss glaze. It also has great working properties.
GA10-D - Alberta Slip Black Cone 10R
You can make a black glaze at cone 10R using only 1% black stain in a 100% calcine:raw mix of Alberta Slip
GR10-B - Ravenscrag Cone 10R Gloss Base
Cone 10 Reduction glaze made using 90% Ravenscrag Slip.
G1947U - Cone 10 Glossy Transparent Base Glaze
Reliable widely used base glaze for cone 10 porcelains and whitewares. The original recipe was developed from a glaze used for porcelain insulators.
G2571A - Cone 10 Silky Dolomite Matte Base Glaze
A cone 10R dolomite matte having a pleasant silky surface, it does not cutlery mark, stain or craze on common bodies
GA10-B - Alberta Slip Tenmoku Cone 10R
You can make a tenmoku from Alberta Slip by adding only 2% iron oxide and 5% calcium carbonate
L3341B - Alberta Slip Iron Crystal Cone 10R
By adding a little iron to 100% Alberta Slip you can make an iron crystal glaze.
Plainsman Electric Bisque Firing Schedule
Three-step to 1832F