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According to Webster's dictionary:
temper: to mix (clay) with water or a modifier (as grog).
grog: refractory materials ( as crushed pottery and firebricks) used in the manufacture of refractory products (as crucibles) to reduce shrinkage in drying and firing

First we clean the clay of rubble and then add more? The theory being, temper (grog) is stable, the clay is not. A lot of rubble (stones etc) in the fresh dug clay is not, so it needs to be removed. During drying, and especially firing, the clay will shrink and expand to a certain degree. Too much, it will crack and/or break. The temper helps to keep the vessel (clay) more stable during those periods. Some clays don't require temper. Others might need up to 1/3 addition. Each clay can be different. Small pots require none, or less temper, than larger ones. Large pots have a tendency to slump when building. The addition of temper helps counteract this.

The intended use of the finished product has some bearing: vessels which will be in and out of the fire (each time experiencing expansion and contraction) will require more than those used primarily for storage. The addition of temper actually weakens the pot. More and coarser temper should be added to cooking vessels as it takes the heat shock better. Minuscule cracks develop every time the pot is heated or cooled. Temper acts as a stopping point for those cracks.

What to use for temper?
Sand/Grit is usually available. But beware of what makes up the sand in your area. Some, but not all, sands with limestone can at times blow the pots during firing.
Grass, Dried Cow Dung, and other organic materials.
Broken, Fired Pottery (Shards), crushed fine.
Shell. Most all shell will work. One caution, fire or burn the shell first. Unfired shell is less stable than clay, firing stabilizes it.

Offered by Steve.
Source: Primitive Wilderness Living & Survival Skills by John & Geri McPherson, $24.95.