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Common Questions, Kiln
Common Questions about Firing a Kiln
Firing a Kiln
How long do I need to let my students work dry before I bisque fire it?
Answer: Let the work dry until it is bone dry. That is, when the piece no longer feels cool, but rather feels room temperature, because the moisture left in clay will always make the clay feel “cool”. The time it takes to let clay dry varies with the weather and environment where you are working. You will notice your work dry faster on the hot dry summer days and slower during the raining season. Most pieces can dry completely when left uncovered for three to four days.
What is the best temperature to bisque fire to?
Answer: Bisque firing is generally fired between cone 06 to cone 04. Cone 04 is the temperature I most recommend, especially when firing earthenware clay. Firing to cone 04 (the hottest of the temperatures mentioned) allows the impurities in the clay to burn out, preventing problems later on in your glaze firing. If the impurities in the clay do not burn out in the bisque they can later burn out in the glaze firing causing pin holing or crazing in your glazes.
Do I need to bisque fire stoneware clay at a different temperature than earthenware clay?
Answer: No. All clay can be bisque fired to the same temperature and even in the same kiln. Bisque firing is simply the process of hardening the clay so that the piece is less fragile and can be glazed easier, with less chance of glaze defects.
How often do I need to put kiln wash on my shelves?
Answer: Kiln wash should always be applied to new shelves. After the initial coat you simply need to up keep your shelves. Grind or scrap off any glaze drippings that occur after your firings and coat the area with new kiln wash. If you find that your shelves are beginning to look as though the kiln wash is thin apply a new coat of fresh kiln wash.
How thick is too thick in the walls of my clay projects?
Answer: The problem with thick walls in clay projects is that it takes longer to dry. When a piece is not completely dry and is put into the bisque kiln it will most likely explode and/or crack. Water (or the moisture in the clay) when heated will expand and turn into vapor, causing the piece to make way for the expansion of the moisture and explode. When possible hollow out pieces that are solid. This makes the object lighter and safer! Any piece greater than ¾” – 1 ½” thick in any area is a candidate for explosion if it is not dried carefully.
When loading the kiln, can the pieces touch one another during the firing?
Answer: Pieces can touch each other and be stacked carefully in the bisque firing only. During the glaze firing, the actual glaze on the pieces will become molten and can run into one another if the pieces are touching in the kiln. When loading a glaze kiln it is wise to place the pieces a fingers width apart to prevent glaze disasters.
How soon can I open the kiln after the firing is complete?
Answer: If you have a pyrometer on your kiln to tell you the inside temperature of the kiln, you can easily know when the kiln has cooled off completely. It is recommended not to open your kiln until it has cooled to 100 degrees Fahrenheit or cooler. It is important to remember to never open your kiln while it is still on or glowing inside. The internal temperature of a kiln can reach very dangerous temperatures that can cause tremendous harm if exposed. If you do not have a pyrometer on your kiln, it is good measure to wait at least a day before opening your kiln.
Thank you for explaining that you should fire students projects when they are bone dry, or when they are no longer cool to the touch. I wonder how companies deal with firing their products, and if they have to wait the same amount of time. I am really interested in this now, I feel like it’s just not something you hear about every day.
Production companies handle this in a few of different ways. Some formulate their own clay that will handle faster drying and firing times. Other companies may load work while still damp and just add a long preheat to finish drying the work in the kiln before ramping up the heat. Other companies have drying cabinets that circulate warm air around the work to finish drying pieces quickly and evenly before loading in a kiln. And then the really large companies like brick plants and Kohler, have train kilns where the work is slowly run through a long kiln that gets hotter and hotter as it moves toward the middle of the kiln and then cools on its way out where it is then reloaded for its next trip around.
My pottery touched during firing and fused together. Is there any way to break them apart without breaking the pieces?
It really depends on how much area is sticking together. If it is just a tiny spot, there is a good chance you can break them apart and refire them to smooth out the rough spot where they were attached . Other times when things stick and are broken apart one piece takes all the glaze with it, leaving a bald spot on the other piece. In that case, it can be possible to dab some glaze in the bald spot and refire to hide the damage. If you would like more specific recommendations on your situation feel free to send pictures to trinityceramic.com
I understand why stoneware and earthenware can be bisque fired together, but I wonder if you can bisque fire porcelain in the same kiln as clays that contain iron? Will the iron get into the kiln atmosphere and discolor the porcelain? (and Hey, Connor!)
Hi Sarah! We recommend that all clays be bisque fired to 04 but whatever temp you bisque to, the clays will not affect each other in the slightest. You can bisque low fire clays, high fire clays, red clays, black clays, and porcelains together. It’s just the glaze firing where you want to match the glaze you use to the maturing temp of the clay. Even then, you can fire porcelains and dark clays together without them affecting each other; it’s just certain glazes that can affect each other if they are close together in the kiln.