|Monthly Tech-Tip |
A kiln firing schedule where temperature is eased to the top, then dropped quickly and held at a temperature 100-200F lower.
Rather than soak (or hold) a kiln at top temperature during a firing, the concept with a drop-and-hold is to approach the top temperature slowly (and only hold for a few minutes) and then drop quickly (by 100-200F) and hold the temperature there instead. At that temperature the increased viscosity of the melt is able to overcome the surface tension that maintains surviving bubbles. The result is smoother, defect free services. Although your glaze manufacturer may not indicate that this type of firing is helpful to finished product quality, your clay body manufacturer may not agree.
These schedules are often accompanied by a continued controlled drop in temperature through the phase where the melt solidifies (e.g. down to 1400F). This improves the appearance of those that variegate and crystallize (but can increase matteness).
The temperature of the hold is often a matter of experimentation. Glazes with plenty of melt fluidity can stay remarkably fluid for hundreds of degrees below the firing temperature. The lowest possible hold temperature at which the glaze has sufficient fluidity to heal defects is best.
The benefits of this type of firing schedule make it worthwhile to learn how to program your own kiln controller. Controllers can remember schedules you enter so you only have to do it once. At the same time, we recommend becoming cognizant of the accuracy of your controller by verifying it with cones. Manually-entered programs are the only way to compensate for this inaccuracy (for example, my controller thinks that cone 6 is 2232F, but 2200F is what drops my self-supporting cone to the right angle).
Drop-and-hold schedules need to be accompanied by other best practices for success. For example, make sure ware is completely dry. Apply glazes as evenly as possible (usually by dipping). Maintain your dipping glazes at a practical specific gravity and viscosity. Use an engobe over the body for functional surfaces that will have a white or clear glaze finish.
Both pieces are the same clay, same glaze. The one on the left went to cone 4. Notice how full of holes and bubbles the glaze is. The one on the right went to cone 6 using the C6DHSC firing schedule. It is perfectly smooth and glassy.
Pinholing on the inside of a cone 6 whiteware bowl. This is glaze G2926B. The cause is likely a combination of thick glaze layer and gas-producing particles in the body. Bodies containing ball clays and bentonites often have particles in the +150 and even +100 mesh sizes. The presence of such particles is often sporadic, thus it is possible to produce defect-free ware for a time. But at some point problems will be encountered. Companies in large production need to have fast firing schedules, so they either have to filter press or wet process these bodies to remove the particles. Or, they need to switch to more expensive bodies containing only kaolins and highly processed plasticizers. But potters have the freedom to use drop-and-hold or slow-cool firing schedules, that single factor can solve even serious pinholing issues.
These terra cotta clays were bisque fired at cone 04 and glaze fired to 04 using the 04DSDH schedule. The glaze is G1916Q, an expansion-adjustable cone 04 clear. That schedule alone is often enough to get transparent, defect free glazes in many situations. But not in this case. The solution was to add a fining agent. In this case we added 2% red iron oxide (to the top glaze). The particles of iron floating in the melt acted as a congregating points for bubbles, helping them to escape. And we got a bonus: a more interesting aesthetic. We did further tests and determined that a 1% addition also worked, but not as well. And screening out the larger particles slightly degraded the fining performance. So we have settled on 2% iron and screening the glazes to 100 mesh. Although iron works here, it will not always do so in other situations. And, other fining agent agents we have used at cone 6 do not work in this situation (e.g. 2% Zircopax, Alumina).
An extreme example of blistering in a piece fired at cone 03. The glaze is Ferro Frits 3195 and 3110 with 15% ball clay applied to a bisque piece. Is LOI the issue? No, this glaze has a low LOI. Low bisque? No, it was bisqued at cone 04. Thick glaze layer? Yes, partly. Holding the firing longer at temperature? No, I could hold this all night and the glaze would just percolate the whole time. Slow cooling? Close, but not quite. The secret I found to fix this was to apply the glaze in a thinner layer and drop-and-hold the temperature for 30 minutes at 100F below cone 03. Doing that increased the viscosity of the glaze melt to the point that it could break the blisters (held by surface tension) while still being fluid enough to smooth out the surface.
I document programs in my account at insight-live.com, then print them out and enter them into the controller. This controller can hold six, it calls them Users. The one I last edited is the one that runs when I press "Start". When I press the "Enter Program" button it asks which User: I key in "2" (for my cone 6 lab tests). It asks how many segments: I press Enter to accept the 3 (remember, I am editing the program). After that it asks questions about each step (rows 2, 3, 4): the Ramp "rA" (degrees F/hr), the Temperature to go to (°F) to and the Hold time in minutes (HLdx). In this program I am heating at 300F/hr to 240F and holding 60 minutes, then 400/hr to 2095 and holding zero minutes, then at 108/hr to 2195 and holding 10 minutes. The last step is to set a temperature where an alarm should start sounding (I set 9999 so it will never sound). When complete it reads "Idle". Then I press the "Start" button to begin. If I want to change it I press the "Stop" button. Those ten other buttons? Don't use them, automatic firing is not accurate. One more thing: If it is not responding to "Enter Program" press the Stop button first.
These are 10 gram balls of four different common cone 6 clear glazes fired to 1800F (bisque temperature). How dense are they? I measured the porosity (by weighing, soaking, weighing again): G2934 cone 6 matte - 21%. G2926B cone 6 glossy - 0%. G2916F cone 6 glossy - 8%. G1215U cone 6 low expansion glossy - 2%. The implications: G2926B is already sealing the surface at 1800F. If the gases of decomposing organics in the body have not been fully expelled, how are they going to get through it? Pressure will build and as soon as the glaze is fluid enough, they will enter it en masse. Or, they will concentrate at discontinuities and defects in the surface and create pinholes and blisters. Clearly, ware needs to be bisque fired higher than 1800F.
Designing a good kiln firing schedule for your ware is a very important, and often overlooked factor for obtained successful firings.
Pinholing is a common surface defect that occurs with ceramic glazes. The problem emerges from the kiln and can occur erratically in production.
Suspended micro-bubbles in ceramic glazes affect their transparency and depth. Sometimes they add to to aesthetics. Often not. What causes them and what to do to remove them.
Plainsman Cone 6 Slow Cool (Reactive glazes)
Cone 6 Drop-and-Soak Firing Schedule
Low Temperature Drop-and-Hold
Manually program your kiln or suffer glaze defects!
To do a drop-and-hold firing you must manually program your kiln controller. It is the secret to surfaces without pinholes and blisters.
Plainsman Zero3 Glaze L,K,H and Firing Schedule