Pit firing is the original method for “baking” clay. It dates back nearly 30,000 years ago. This process is done typically in a hole in the ground, or a pit, pots are placed in the pit and burned. Pit firing is an atmospheric process all of the colors and patterns are derived from the process and what is consume in the fire. Items that are burned will turn to vapor and will swirl around the pieces in the pit. If the pieces are hot enough to have their pores open the colored vapor will enter the pore and stay there, if not pot will not have color besides black, gray, or white. There are several variations in which to do a pit fire, this will address Up in Smoke Pottery’s method. Feel free to contact us if you have any questions on our process.
We start out with a hole in the ground, that is at least 18” deep (depending on the size of what we are firing, I’d have at least 10” deeper than the largest pot). It can be lined with stones or concrete, our current pit is not. Lining with stones or concrete has some benefits, but some hazards as well. The lining does help insulate the pit, raising the internal temperature slightly, also lining with fieldstone can produce other flashes of color. Down side, the liner cracks and breaks and sometimes explodes breaking everything in that part of the pit.
The 1st layer is wood shavings, or sawdust, or wood chips. You can also use dried grass clippings and dried leaves, but it does change the colors since you have changed the ingredient. Into the sawdust we add the colorants, typically the powder ones such as copper carbonate or red iron oxide. We then add the pieces either terra sigg’d or burnished. I recommend sigging, but for you purists knock yourselves out burnishing.
We then may add additional powdered colorants around various pieces if we see a need in certain areas. We then add the more solid colorants such as banana peels or manure. Once we have the colorants in place we add the newspaper layer. Click here for a list of colorants.
The newspaper layer serves a few purposes. One, it is highly combustible and gives an easy method for starting the pit. Two, it also does a great job of spreading the fire across the entire pit so it all goes up at once versus starting at one side and going across (IMPORTANT). Three, acts as a cushion as you add the wood on to the fragile pottery. Yes I do bisque first, it is easier to transport the 30 miles to our pit and improves our survival rate. Back to the newspaper, we lightly crumple the paper a couple sheets at a time. We cover all the pieces in the pit with a nice layer of newsprint ensuring couple inches over every pot.
The wood is comprised of a mix of hardwoods; we will use soft woods, but must be very dry and small amounts. You want to avoid any wood with paint or excessive glue, avoid any manufactured wood such as chip board, OSB, Plywood, etc. The majority of our wood comes from old pallets, any business that ships regularly will have broken pallets they want to get rid of, and those who don’t ship that often usually don’t want the pallets they have. We cut down the pallets to manageable pieces, don’t worry about the nails, they’re colorants! Carefully position the wood in a spider-web-like fashion, with each board crossing over several pots to distribute the weight. Keep doing until you have a solid structure to pile wood on, but first fill in all the voids between the supports once it is complete pile on the remaining wood. The wood layer needs to be even to get a properly dispersed coal bed, and the spider web of wood supports the weight. Ours once supported a fully grown Holstein with only minimal breakage!
How much wood? For our pit that is 7’ x 6’ x 3’ we used 9-10 pallets plus some scraps. You want enough wood to reach a high enough temperature to get color and your spider-web needs to support all the wood. You do not want to add wood as it is burning; one, adding more fuel actually lowers the temp until the piece heats up, and you don’t want to do that if it is the moment the vapors are swirling and two, you will most likely have to throw the wood on thus risking breaking a piece.
Once the pile is built you can light it on fire, trick is to leave a few spots of paper showing or make paper fuses it is not too late to add them now, this is where we typically remember to add the fuses.
The paper layer should allow for the fire to travel across the pit and it will not take long before you have a raging fire. Let’s take a minute to talk about pit safety. Hopefully you have read these instructions in their entirety prior to doing your 1st pit fire, if not there is always the fire department. Make sure there is no grass or other things that can catch fire around the opening of the pit. We typically dig around the pit 18”-24”. Have a something to put out any unwanted fires, such as a water source, buckets of water, or a shovel. Fire is hot and will burn you if you come to close; items exposed to fire will be hot and may burn you! That is my McDonald’s clause.
The fire will burn down to a manageable level that you can watch from a distance after about 20 minutes, you should start to see pots poking through the ashes. They will appear white at first and the colors will appear as they cool and the air hits them. At this point you can cover the pit if you wish, we choose to leave our pit open. Historically, it was covered and allowed to simmer so to speak, we have not noticed a difference in the colors or breakage. Pit firing is high breakage event; it is not uncommon to have 40% loss due to breakage.
When the pit has cooled, typically the next day, open it and examine your masterpieces. Take care removing the pots as there may be areas of heat and “items exposed to fire will be hot and may burn you!” You can wash the pieces and seal them with a sealer of your choice.