Even assuming a design that Clouds happen, so the sun is not completely reliable. Thermal Balance Before building anything, it is important to consider the overall thermal balance of the kiln. Assume that the kiln is basically an insulated box with a small hole through which concentrated sunlight is projected. The box has energy gains and energy losses. The energy losses increase as the temperature in the box increases, so the temperature climbs until the energy in equals the energy being lost.
Energy is lost via three mechanisms.
The first is the conduction loss through the walls of the box. This heat loss is equal to the temperature difference between the inside and outside walls multiplied by the thermal conductance of the walls. The second heat loss is the radiant heat transfer through the hole that admits the concentrated solar radiation. For this purpose, we can consider our kiln to be a black body at the temperature of the kiln's interior. The energy emitted from this source is proportional to the 4th power of the absolute temperature, times the area of the aperture.