How much grog for a claybody for wheel-thrown foodware?

[GEP]

If you could choose an amount, how much grog would you add to a claybody, as a percentage of dry materials? This is for making wheel-thrown food ware. 

Also, if you could choose from different sizes of grog, which would you choose? (0-0.2mm, 0-0.5mm, 0-1.0mm) I think I would choose the smallest one, the largest one seems too big, but I’m not sure. 

[Min]

I'ld try 5% of dry body weight and go from there. Just guesstimate the weight of the pugged clay as being approx 20% water and subtract that weight from the wet clay weight and wedge it in, or mix it in some clay slurry reclaim.

Re mesh size, I'ld try 80 mesh (to fines) and also 48 (to fines) and see which works better for what you are doing. 

[liambesaw]

If you're using store bought grog, be careful of increased absorption as well

[glazenerd]

Depends if the grog is being used to increase structural support: sculptural for instance. If used for appearance; then the parameters change. Mullite and molochite are commonly used in red or dark body for "speckling; or manganese/iron in white body or porcelain. The human eyes can see 50 mesh; so 20-50 mesh is used. 1% of 60 mesh manganese for example will produce 400 "speckles" per square inch. Stoneware bodies will handle 0.25 to 1.00% additions with nominal effects on absorption. If your clay body produces a consistent cream when you throw; there should be plenty of fines to encapsulate grog additions. Larger grog particles will produce visual effects; but are also tougher on the hands. I would start at 0.50% addition in a small test batch. Mullite, molochite, Manganese, and magnetite iron are all non-absorptive materials.

[neilestrick]

When adding grit to a clay body, if you add too little it will feel like little impurities. You have to add enough that it feels like it's there on purpose. I would start with 3% and test from there, as high as 10%. As noted before, it will increase absorption so you may have to make other changes to the body to compensate. Commercial terra cotta bodies, for instance, are often rated 2 cones higher when they contain grog.

Personally, I prefer the feel of silica sand. It's smoother, moves in a nicer way, and is easier on the hands. Try some 70 mesh and see what you think.

[GEP]

Thanks for the answers everyone! I was asking this question on behalf of a student of mine who is located outside of the US. Her clay supplier can customize her clay to some extent, including grog size and amount. She is very grateful for the guidance!

[neilestrick]

9 minutes ago, GEP said:

Thanks for the answers everyone! I was asking this question on behalf of a student of mine who is located outside of the US. Her clay supplier can customize her clay to some extent, including grog size and amount. She is very grateful for the guidance!

I'd have her try some of the grogged bodies that they already make and decide from that.

[GEP]

4 minutes ago, neilestrick said:

I'd have her try some of the grogged bodies that they already make and decide from that.

She already tried one and thought the grog was "too fine." The suggestions above point towards the next larger grog size.