Hard Spots in Moulds

Hand pouring of slip into a mould

Some ceramic moulds have small areas where the kiln wash does not seem to adhere as well as on the rest of the mould.  This comes from the manufacturing of these slip cast moulds and this blog post explains how it occurs.  The question is what to do to make the mould separate from the glass after firing.

Coat the mould as usual, which shows up the area where it seems no kiln wash is sticking.  There is some coating the area, but not in the same amount as the rest of the mould.  You can add a little extra kiln wash to the area once first layer has dried, but be careful to avoid creating a ridge against the rest of the kiln wash. If one does appear gentle smoothing with a finger can disguise the transition.

Another approach is to abrade the spot a little to make a more textured surface for the kiln wash to attach.  This needs to be done carefully and by hand to avoid creating a shallow divot in the mould.

The safe approach is to coat as usual and slump a sacrificial piece of glass to ensure the glass does not stick to the hard spot.  If it does not, the spot has enough separator to be useable, although I would continue to add kiln wash to that spot for several firings.