Daubing
The clay mixture usually made by the cupola-man for repairing bad spots in the cupola and for lining the inside of ladles. Too frequently these mixtures are made by those who possess no knowledge whatever of the requirements in the case; the result being that the metal boils when it enters the ladles, and cupolas are prematurely worked out and always amiss for the want of a rightly mixed daubing for keeping them in good working shape. A good daubing for ladles is made from equal quantities of good moulding and fire sands, mixed well together in a dry state, to be afterwards brought to the right consistency for use by the addition of thick clay-wash made from common clay.
Daubing for repairing the cupola should be of a much stronger nature, and, what is of equal importance, should be properly applied and not thrown on the walls carelessly in thick lumps that only fall away into the stock as soon as the extreme heat is brought to bear upon it, leaving the part it has vacated entirely unprotected and very materially impeding the regular action of the cupola.
A good daubing for cupolas is: Fire-clay (not common clay), 50 parts; fire-sand, of a good siliceous kind, 25 parts; and ground fire-bricks, 25 parts--to be mixed well together when dry, and brought to the right consistency with water. This should be rubbed on evenly, as thin as possible; and should the patching require to be more than one inch thick at any part, it is much better to cut some of the backing away and insert new bricks. Some have an idea that salt mixed with the sand and fire-clay for this purpose acts beneficially. I think this bad practice may have originated in a misconception of its right use in the stone and earthenware manufactories, where salt is extensively employed as a glazing agent, its volatility at furnace heat combining with other qualities to fit it for this use. No doubt it has been thought by those who were only partially instructed in these matters that a permanent glaze, similar to that produced on the earthenware, might be imparted to the walls of the cupola by the use of salt; but if they will remember that salt fuses at a red heat, they will at once realize that any salt used for cupola purposes must constitute a slag-forming agent, at every heat, until every vestige of salt has been melted out. See REPAIRING THE CUPOLA; CUPOLA; SEA-WATER.