Refers to a black matte glaze that resembles the surface of iron. These are most easily made at cone 10 reduction.
Cone 10R gunmetal black pure Alberta slip glaze
It has 5% Mason 666 stain added.
Alberta and Ravenscrag Slip pure at cone 5 reduction
At cone 5R pure Alberta Slip (left) is beginning to melt and flow down the runway of this tester. It is producing a matte gunmetal surface. Pure Ravenscrag Slip (right) is just starting (it needs frit to develop melt fluidity at this temperature). The iron in the Alberta Slip is melting it because of the reduction atmosphere in the kiln (it does not move like this in oxidation).
Additions of black stain to Alberta Slip at cone 10R
Alberta Slip (50:50 calcine:raw mix) with 1, 2, 3, 4 and 5% added Mason 6666 black stain. Fired at cone 10R. Semi-gloss blacks are produced. Increasing stain percentage above about 3% does not darken the color appreciably.