Primitive Pottery: Adding Temper to Clay
(As featured in the February 2008 issue of Practically Seeking)
Last month we discussed how to process clay using the Water-Extraction method
and I hope that many of you tried it out and now have some raw material with which to work, because this time around we're going to be discussing The Tempering Process. Now that we've taken all the sand and impurities out of our material it's time to make the clay usable for pottery by adding some temper back in.
All clay, whether natural harvested or commercially produced, is different and as such responds differently when handled, molded, dried and fired.
Two of the most important characteristics with which we need to contend are shrinkage and the ability of the clay to withstand thermal shock. Temper (also called "grog") helps us control both. The goal is to find the correct amount of temper for the clay with which we are working, and for the item we wish to create.
Learn more about Harvesting,
Processing and Firing
Primitive Pottery workshops.
Temper can be bought, found, or made of many different possible materials.
I have made temper from aquarium gravel that I crushed down to an appropriate size, crushed shells, old broken pottery that I have pulverized down into little pieces, sometimes even coarse sand. The one material that I do not recommend using is beach sand, as it is most often too round. Temper must be "edgy", so that the clay can stick into it. The most important thing, no matter what you use, is to ensure that your temper is a consistent particle size, of a consistent material, added in known proportions. (Which is why ya got rid of the other stuff last month!)
Some clays will do just fine with no temper at all, while others will require a fairly large amount. The balancing act comes with knowing that if we add too little temper the pot may crack while drying or firing, but if we add too much, the clay may become crumbly and not hold together well in construction or use. A rule I tend to follow is the more often a container will be subjected to temperature changes (i.e., a cooking vessel) the more temper I will add.
Tips & Tricks for Working With and Firing Naturally Harvested Clay
- Some clays lack good plasticity, but this can be improved by keeping your clay stored in a moist, air-tight state (such as in a closed jar or zip-loc bag) for several months. Adding the commercially available product bentonite can also improve plasticity.
- Warm your pot slowly and evenly before firing, allowing plenty of drying time before it reaches the "boiling" point (212 degrees F.)
- Warming your pot between the fire and a reflector wall will help to prevent uneven heating.
- Look for color change in the pot as an indicator of when to begin ramping up the heat.
- After firing, allow the pot to cool slowly in order to prevent cracking. (Thermal shock can happen then too!) A covering of ash can be used to help insulate the pot as it cools.
Step-by-step Instructions for how to Add Temper to your Clay:
- Remove enough of your moist, pure clay to make a ball about the size of your fist.
- Divide that ball into eight equal-sized portions and form each portion into a ball.
- Flatten each of the balls into disks approximately 1/8" thick. Set one of the disks aside as your "control" portion. You will not be adding any temper to this disk.
- Create a pile of temper that is about the same size, in both diameter and thickness, as one of your disks.
- Mash one of the disks onto your pile of temper, then flip it over and do the same on the other side, as though you are coating chicken with flour. Place this disk to the side, then do the same with each of the other seven disks. As you coat each disk with the temper, place it on top of the others to create a little tower.
- Once all seven disks are coated, smash down on the top of your tower, mushing all the disks together. Continue to work the disks all together into a new, single ball, making sure that your temper gets distributed evenly throughout. Your clay now has about a 10% temper ratio.
- Divide this newly tempered clay into eight equal portions, roll each into a ball and flatten them into disks just like you did with the last ones. Take one of the disks that is about the same size as your untempered "control" portion, and set it aside.
- Make another pile of temper and repeat the process of adding the temper to each of the other seven disks, mushing them all together and making sure this additional temper is evenly distributed through the ball of clay. You have now raised your temper to approximately 20%
- Repeat this process three more times, always remembering to set one of the disks aside before you begin. Each time you are increasing your temper by about another 10%. In the end you will have five disks: one with no temper, one with 10%, one with 20%, one with 30%, and one with 40%.
- Now form each of your disks into a shallow bowl. If you notice the clay cracking as you work it into shape, don't worry. Just put some water on your fingers and work it on to the cracked areas until you have added enough moisture back into the clay that it no longer cracks. Make sure to keep your disks in order! Using a twig, nail, or other sharp object, label your bowls 1 through 5 (or 0 through 4) so you will know which bowl contains what amount of temper.
- Leave them alone for a few days and watch how each disk responds to drying. If you want to speed things up you can treat them as you would a pot, and slowly warm them up near a fire. Once they are completely dry, check which disk or disks have the least amount of cracking, breakage or crumbling. That is the percentage range of temper that will be best for your clay.
(If you find that there is not enough difference between the dried disks you can go the extra step and fire them, as we did with the disks below. After firing it became very obvious that disks 2 and 3 gave us our best results! )