Tempering Pottery Clay
It’s an oft repeated scenario. A person finds some clay somewhere, then comes home excitedly and makes a pot from it. Disappointment soon follows when the pot dries and breaks into pieces. This is what happened to me when I first found clay in the wild, after my pot broke, my high school art teacher explained to me the importance of temper in pottery clay.
Temper is non-plastic material that is added to clay to keep it from cracking when it dries. It is most often sand, ground stone or ground fired ceramics but historically a wide range of materials have been used for temper.
Why Clay Needs Temper
When clay gets wet it expands and when it dries it shrinks. Natural, unprocessed clay is usually a very dense material, this density prevents water from easily moving through it. So when clay in nature dries usually the outside dries but the inside stays wet causing some areas to shrink while others stay expanded, this is what causes clay to naturally break apart like the crackling you see in the bottom of dried mud puddles. If you don’t add temper to your clay any pottery made with it will crack up just like that clay in the bottom of a mud puddle.
Temper adds porosity to a clay body, thereby allowing water to move more easily through the clay. This promotes more even drying and thereby more even shrinkage, reducing cracking and breakage while the pot dries and shrinks. The porosity in the clay body also allows water vapor to easily escape when the pot is being fired and protects the pot from breakage caused by thermal shock when the temperature rises or drops suddenly. So temper can protect pottery from cracking during the drying and firing stages.
All clay used for pottery needs some amount of temper to be usable. Clays that do not need temper added to them are those in which some amount of non-plastic material occurs naturally and so already possess the needed porosity. These clays are called “self-tempered” clays and they are very rare in nature. Notable self tempered clays in the Southwest include that used to make Hopi Yellow Ware and the clay used to make Ramos Polychrome near ancient Casas Grandes in Chihuahua, Mexico.
Kinds of Temper
In the ancient Southwest sand was the most common temper material used in the south of this region. In the northern Southwest crushed pottery was the more common temper to use in the past. Most store bought clay today contains grog as the temper, grog is just fired clay that is ground and screened very fine. Sand is easy to use because it is very abundant and can be collected almost anywhere but the particles in sand usually have rounded edges which makes it less effective as temper. Crushed pottery makes a better temper because the particles have sharp edges that the clay can grip more easily but it requires more work to prepare than sand.
Ancient people worldwide used all kinds of things for pottery temper including;
- Ground stone
- Ground ceramics
- Volcanic ash
- Diatomaceous earth
- Plant fibers
- Much more…
If you want to try some of the organically derived tempers mentioned above, remember that they will burn away in the firing process resulting in very porous pottery.
Following are a few temper materials that I have used and can recommend.
Sand: Sand is really convenient to collect at any creek but usually has rounded edges just like river rocks. You can mitigate this by grinding your sand before you use it if you want. Pass your sand through a screen to remove larger particles and sticks or any other unwanted debris.
Ground stone: I have used ground stone a lot in my pottery, it provides those sharp edges that make it work well as temper in clay and I can grind it to the desired fineness. Some rocks are very hard and difficult to grind, the potters at Zia Pueblo use ground basalt which is extremely hard to grind. My favorite is soft and decomposing granite which is easy to grind very fine and makes a great temper material.
Diatomite: Also known as diatomaceous earth, is the fossils of ancient algae. It naturally has sharp edges and is almost pure silica so it makes a fantastic temper. I have naturally occurring diatomite in my area but not everyone may be so fortunate. If you don’t mind using store bought material, you can purchase sacks of diatomaceous earth at most hardware stores.
Volcanic ash: Another somewhat rare material that makes a great clay temper is volcanic ash. I have trouble finding this near where I live but up in northern New Mexico many of the pottery producing Pueblos use volcanic ash. The video at the top of this page includes footage of the great Maria Martinez collecting volcanic ash to use in tempering her clay.
Temper materials that have very fine particle size such as volcanic ash and diatomite can be used in pottery that will be sanded smooth when it is bone dry. While tempers with larger particles like sand and ground stone will leave temper particles standing out of the pot surface after sanding.
How to Mix Temper Into Clay
The amount of temper needed varies depending on the clay used and the individual potter’s working style and preference. I usually use 20% as a good starting point for trying new clays, this amount will make most clays resist cracking and able to handle the temperature fluctuations of the outdoor firings I perform. Adding 20% temper is easy to measure out using a 4 to 1 ratio, just combine 4 measures of clay with one measure of your temper material.
I like to process my clay dry because it makes mixing the clay and temper much less work. With the dry clay/temper mixture in a bucket, I can just pour it back and forth between two buckets a few times to easily mix the temper through the clay. If the clay is already wet then it will require a lot of kneading or wedging to mix the temper through the clay evenly.
You may find that 20% temper is too much or maybe that it is not enough, but how will you know?
Adding temper reduces the plasticity of clay, so if you find that your clay is not plastic enough you may want to try reducing the amount of temper you add. If however after adding temper you are still having trouble with your pottery cracking then try adding more temper. You will probably need to experiment to find out how much temper is needed. Try not to go much over 30% temper or your pottery may become fragile and unworkable.
Many people who use wild clay spend a great deal of time purifying the clay either by screening or levigating. But the material that they are working so hard to remove is non-plastic material, the same as temper. So it might be more efficient to leave that material in place as naturally occurring temper, grind the clay so that it is all of a fine consistency, then add more temper as necessary. Less labor while taking maximum advantage of natural temper.
Would you like to learn more about processing wild clay? Here are a few resources to help you on your journey.