Splitting glass blocks into two plates by thermal stresses - where block is first cooled and then heated so it cracks along score mark

A score mark or groove is made on the periphery of the block and between its two end surfaces to produce a zone of weakness. Cooling (a) is then used to create thermal stresses in the scored block, so the latter cracks along the score mark leaving only a small uncracked zone, heating (b) on the block is next used to complete the crack, thus forming two glass plates. Cooling (a) is pref. achieved by immersing the block in a bath of coolant, then allowing the block to return to room temp. in ambient air, or immersing the cold block in a water bath (b) at or above room temp. A clamp is pref used to apply compressive stress at the top corner of the block while the opposite corner is immersed in bath (a); the clamp is removed after cooling (a) and before heating (b). Used e.g. in mfg. optical components, where no expensive tools are needed to cut the block.