"The furnace used by the writer for making such tests had nearly the form of a puddling-furnace. One-half of it was built of one mixture and the other half of another, running through the furnace from end to end. Bridge-wall, roof, side wall and neck would show how the brick stood in each posi tion. From the results of the tests a fair comparison could be obtained of the value of the brick. The draft was a direct one to the foot of a large chimney, and the coal used was a mixture of the best Cumberland coal and our own. A brick of the mixtures used in building the furnace was taken as a standard. One of these brick, with another, either of some other mixture, or some brick which we wished to test, were placed side by side in the neck of the furnace, which was then fired as hard as possible for a certain length of time. When the furnace had cooled off the brick were removed, and the effect carefully noted, particularly as regards shrinkage and vitrification, and the effect of the heat on the furnace was also noticed. The
heat in 36 hours was intense enough to vitrify any brick, but not enough to destroy them.
"As the demand now is for a hard-burned brick, the diffi culty of spotted brick arises. These are brick which appear to be of poor quality, for though a clay may not contain more than i per cent. of oxide of iron, yet if it is exposed to great heat these spots will show ; and at present, buyers, with the exception of a few who have learned their value, will not take spotted brick. All the brick from the other places, which the writer has tested, will, when exposed to our greatest heat, show some spots, although as they come out of an ordinary kiln they are free from spots.
"The two peculiarities which have made the Mount Savage fire-clay so famous are its freedom from impurities and the fact that this clay contains such a proportion of silica to the alum ina that the brick, after they have been hard burned, will swell a little instead of shrinking, no matter how much they are heated."