Can you controll the temps in the kiln?
step 1. Load it up an pack it tight. no big gaps.
step 2. Set the kiln to 240* let it soak at 240* for 12 to 24hrs..
Step 3. ramp it up t 400* let it soak for 8 hrs then ramp it to 600* let it soak for 24hrs.
Step 4. turn off the kiln and give it 24hrs to cool off.
Step 5. test the rock if its not slick then do it again butyou can skip step 1 and 2 the 2nd time because the rock s dry now.
Good luck and let us know how it turned out.
PS. if you Have any 10 inch +, pieces let me know might like to do some trading with ya.