I laid some hearth tiles last week on a concrete hearth. They needed soaking prior to laying on heat resistant adhesive. Today I went back to grout the tiles, and as I applied the grout the tiles started to crack, or, more to the point, the glaze was cracking as I worked my way across !! Anyway when I cleaned them up the cracks in the glaze wasn't so apparent being only hairline. Any ideas why this would happen ? I've laid acres of tiles in my 40 odd years but never had this happen before.