Just had a few questions really

1) Finery Forge: it used to take only 1 lump of coal to power now it seems to take 2 to complete a batch of 9 cast iron. Is this a repair issue or a bug?
2) Repairing: if lets say I have a Q20 forge and I use a q10 brick to repair would that impair the Forges quality? What about other repairables? (palisades, ovens, kilns, anvils etc)
3) Clay spots: how long does it take for an area of clay to regenerate its quality? Also, I seemed to have exhausted a spot of q10 clay, will that regenerate over time or not? :-S
4) Does having an equipments "inventory" open (right clicking an oven/kiln/crucible so that the door is open) affect it's cooking/smelting time?
Many thanks!