From the good people at Ibstock Hathernware.

Regarding the use of Sugar or Molasses to aid glaze adhesion. This would have
been to make it easier to handle the product prior to firing without wiping
off the glaze. Adhesion is also important during the early part of firing
when gases and steam are given off by the units. The sugar/molasses would
have 'burnt off' at between 5-600F and we now fire up to 2400F.