Here is a link for the furnace cement:
1st thing to do with the metal washing machine tub or the empty barrel is to cut a hole about 2 inches from the bottom to insert a burner pipe. Next line the tub or barrel with half to one inch with furnace cement. Make sure you do not fill in the hole for the burner. The purpose of the cement is 2 fold. 1st it will heat up the water faster and 2 it will help retain heat much longer. Or you could cut the bottom out completely line the drum or tub with furnace cement and use blocks to keep it above the burner. Once you turn the burner off you would need to put a lid over the pot to retain more of the heat.