Are Holes Needed in Stainless Steel Moulds?

 “Do you drill holes in the bottom of the stainless steel moulds the same as with the ceramic ones? I imagine so, as the air issue is the same?”

When draping over stainless, holes are not required unless there is a depression at the highest point.  It is debatable whether required even then.  The steel is expanding more than the glass during the heat up and contracting more on cool down.  This effect means there is sufficient space for any air to escape.

In slumping moulds, stainless needs to have a significant draft to avoid the steel trapping the glass during its greater contraction during the cooling.  The combination of the draft and the greater expansion during heating allows air to flow from under the glass, unlike ceramic where the glass is the faster expanding material.  The greater expansion of steel leads to less chance of the glass sealing to the mould and creating bubbles. 

However, there is no harm in being cautious by drilling small holes at the last places the glass will touch down.  These usually are at the join of the curve and the flat bottom.  The glass will touch down first in the middle of the bottom, so no hole is required there.