Grog is just ground up pottery so equivalent to sand in particle size so about the same, but with better heating properties, although it won't be cheap for the quantity you will need. You'd have to test it... you can do a jar separation test where you shake up some of the clay in water and leave it overnight to settle, the bigger particles sink faster and the clay stays suspended in the water longer... next day (sometimes much longer depending on the clay content) you will see distinct layers and you get a good idea of the component content.
I'd be interested in the results tho if you do go for grogged pottery clay. Its often considered but I don't recall anybody using it in the end...
Having looked at grog (way more than I would like) you can get multiple grades of grog.
Might be worth asking what the minimum alumina content of the grog is (if it's made of old crushed firebricks; you may be pleasantly surprised to find that the grog not only helps with shrinkage... but alsso has refractory properties).
Also, grog can be ground to a range of sizes - from as fine as fire clay all the way to 6mm-10mm particles. The finer grog works better in smaller parts (e.g. small joints between bricks) and a blend of fine-coarse will work in large jobs (like casting a dome).
Price wise, the best I've found is £12 for a 25kg bag; with flat rate delivery for £25.50 for a pallet up to 1000kg (or £8 for a single bag)
Thanks for responses. I will give it a test and let you know. It costs me £3.50 for 10kg but I think there's more like 11kg / 12kg in each bag. Basically it the reclaimed (left over) at the end of the pottery sessions. So in a nut shell.. Do I mix with sand or not ?? Thanks again
Hi. A late post re the general topic. If it is a groged clay for potters it will be a fired clay grog with a max particle size of probably 4mm to 6mm. To check if you need sand do a shrinkage test. Make up a 800mm X 50mm tile of at least 25mm thickness .... measure the 800mm exactly. Dry it.... with gentle heat will be OK. When it is dry measure the 800mm length..... every 2mm of reduced length will be equal to 1% of shrinkage. If the shrinkage is more than a couple of % then you will need to add sand till you get a minimal shrinkage.