What coating will stop copper oxide adhesion to steel furnace

I am in the process of developing a furnace for oxidizing cuprous oxide to cupric oxide.The current model operates by dropping cuprous powder into a superheated airstream. The problem is that the resulting product tends to adhere to the steel walls of the furnace at temperatures in the range of 900-1100F, a situation which compromises the efficiency of the furnace. I am looking for an explanation of why this happens and advice on what type of coating material on the inside of the furnace would minimize or eliminate this effect.

Thank you.

Can you brief by which process you are making Cuprous oxide, i.e. Air Oxidation or electrolytic. Specification of the muffel are you using. Your method of loading the cuprous oxide in the vertical furnace & the quantity, i.e. is the material allowed to fall free. Copper , Cupric oxide, Cuprous Oxide all have the tendency to stick to steel wall at high temperature, various measures have to be taken to prevent it from sticking.