It is almost impossible to burn the contents of any kiln quite uniformly throughout, and the clinker will be found differing in colour accordingly.
It should not be clinkered into large masses - should rattle well as it comes out of the kiln - should be honeycombed in texture and nearly free from dust.
Some will be found of a bright yellow colour, and of light specific gravity. This will set very quickly, and it should be picked out.
Some, again, will be of a fink or dirty white colour, but more or less heavy according to the heat to which it has been subjected. Clinker of this kind has been imperfectly burnt, and must be again passed through the kiln. It is a dangerous substance to use.
Dense glazed black clinker indicates excess of lime, and will also yield l General Scott in RE. Papers, vol. xi dangerous cement inclined to blow; dark blue clinker, a sluggish cement; and a brown clinker with much dust, a weak cement.1