For practical purposes most people ash at 600 - 650C overnight in an electric muffle furnace, using a gas fired furnace will lead to losses due to the light ash blowing out the furnace exhaust.
The most important aspect of ashing is the depth of the carbon bed, if it is deeper than around 1 cm you will blanket the carbon with ash and the reaction will need rabbling of the bed at intervals to get complete ashing.
The door of the furnace is cracked open about 1 cm to allow the air access needed for ashing to proceed.
Always best to use coarse piece activated carbon as per what the gold industry uses, fine powder type carbon will adsorb gold quickly but is very prone to blanketing during ashing.
Asking is usually done in 316 ss trays with edge turn ups of around 1 cm.
You can layer the trays keeping about 40mm spaces between trays, use 25mm ss tube lengths as spacers at each corner of the trays.
Deano
