pottery question

[mcginnis6010]

I dug some clay from a clay deposit near my house. After sifting it I tested it for elasticity and it worked fine with minimal cracking, however, its a very sandy clay and I was wondering if anyone can tell me if this is fine or if i need to add temper or something to it. I'm using local crushed shell from a nearby river for temper.

[Zuma]

I used a sandy clay without grog and it fired fine. Alot of native pottery has crushed quartz and other minerals.
There is a thread about crushing shells and roasting them.
There are different grogs for different uses.. Things like strength, heat transfer and plastisity. If you are going to try a reproduction see if you can find out what the natives used.
Zuma

[mcginnis6010]

Thanks Zuma I'll fire a test piece and see what happens. The clay I use comes from the Shenandoah Valley near the Frontier Culture Museum in Staunton.

[mullet]

A lot of times when someone discovers a skeleton, they can tell it is Native American by how wore down the teeth are from the sand in their diet.

[Jodocus]

The 3 natural clays I've tested throughly so far are very different. One needs no temper at all if care is taken while forming, drying and firing. It's from oxford-marl layers, light leather colour when fired.
On the other extreme, there is a clay from the septaria layers of the oligocene. It will crack in the drying phase already for sure if I do not temper it massively (twice as much sand as dried clay). If so, however, it is great stuff, there was a whole industry based on it in the burgundian time.
In between these, there is a loessy clay that I use mainly cause it's nicely red from iron.The oxford-clay will also dry much much faster than the septaria clay, this is already a sign it binds much less water. But it's nowhere near as hard when dry.

Of course, shrinking is not the only issue you would adress with temper. Also, large vessels will hold their shape much better during the building process if the clay is tempered coarsly (you still should add finer material, too, for to occupy the space between the larger particles.) Go for roughly half the wall thickness as size for the larger particles. Avoid round particles, prefer edgy shapes.

In contrast, polished surfaces will get much better when no or very fine temper is added. Silt, like loess, is good then, it reduces the amount of water bound in the clay while still allowing for a smooth surface. But it will not help you stabilize the walls while building.

Also, you can add fibers, I like horse manure best (not fresh). They will greatly strenghten the clay when it is wet, and you can build stuff that would be impossible without fibers in the clay. Also, stuff will be as good as flameproof AFTER firing. Any organic matter can be used besides fibers: bone, coal, wood. I've not extensively tested these yet, however. Organically tempered pottery will be lightweight, and porous, soaking up lots of water, but will usually not let it drip through. Usually, organic temper reduces cracking from shrinkage, but not the shrinkage, so you may still add mineral substance.

In the end, there's no way around testing anyways 

[mcginnis6010]

Thanks for thevinfo ill put it to good use 

The 3 natural clays I've tested throughly so far are very different. One needs no temper at all if care is taken while forming, drying and firing. It's from oxford-marl layers, light leather colour when fired.
On the other extreme, there is a clay from the septaria layers of the oligocene. It will crack in the drying phase already for sure if I do not temper it massively (twice as much sand as dried clay). If so, however, it is great stuff, there was a whole industry based on it in the burgundian time.
In between these, there is a loessy clay that I use mainly cause it's nicely red from iron.The oxford-clay will also dry much much faster than the septaria clay, this is already a sign it binds much less water. But it's nowhere near as hard when dry.

Of course, shrinking is not the only issue you would adress with temper. Also, large vessels will hold their shape much better during the building process if the clay is tempered coarsly (you still should add finer material, too, for to occupy the space between the larger particles.) Go for roughly half the wall thickness as size for the larger particles. Avoid round particles, prefer edgy shapes.

In contrast, polished surfaces will get much better when no or very fine temper is added. Silt, like loess, is good then, it reduces the amount of water bound in the clay while still allowing for a smooth surface. But it will not help you stabilize the walls while building.

Also, you can add fibers, I like horse manure best (not fresh). They will greatly strenghten the clay when it is wet, and you can build stuff that would be impossible without fibers in the clay. Also, stuff will be as good as flameproof AFTER firing. Any organic matter can be used besides fibers: bone, coal, wood. I've not extensively tested these yet, however. Organically tempered pottery will be lightweight, and porous, soaking up lots of water, but will usually not let it drip through. Usually, organic temper reduces cracking from shrinkage, but not the shrinkage, so you may still add mineral substance.

In the end, there's no way around testing anyways