How are you cooking it? a kiln , toaster oven , turkey roaster or in the ground ? And it depends how thick the spalls are , but 550 and 3-4 hrs hold time is usually good. I need a kiln bad , but my best method is abo style dig a small depression and lay the spalls out not touching each other and make sure the tops of all the spalls are level with each other ,then cover with about 2" - 3" of DRY sand and pack the sand on top and then spred a 20# bag of charchol even across and light and "set it and forget it " . and allow for a day to TOATALLY cool down slowley with out disturbing the sand or blanks ' i know its hard' . When done they shoulb be good and glassy with color change . If some didnt treat all the way its ok you can flip them and add them to the next batch rather than blowing up good rock by getting it to hot , Hope this helps - john