Scientific American 27, 27.3.1847
It is the case in this as in other arts and trades, that different workmen have different modes of operating, in some particulars; though on the same principle. In the process of japanning coarse and indifferent articles, as practised in some hardware manufactories, boiled linseed oil is the only blacking material used. The oil for this purpose is first boiled for several hours, and until, on cooling, it will assume the consistence of varnish. This oil is brushed in a cold state, over the work, which is then placed in the open air till the oil has begun to become adhesive. It is then placed in an oven prepared for the purpose, (a common cooking-stove oven will answer) and a gentle heat applied, but not raised above 300° until the oil has become nearly fry; after which the heat may be increased gradually until the oil becomes a full black. It is then withdrawn and allowed to cool gradually. For more delicate work, instead of oil, a solution of gum shellac in alcohol is used, and managed in nearly the same manner, the process being varied according to the nature of the articles, the construction of the oven, &c., as dictated by the experience of the operator. A japan varnish for this purpose may be made by boiling shellac in oil; or for an extra dense black, a solution of asphaltum in spirits of turpentine may be used, or a compound of all these ingredients together. - But whne asphaltum and spirits are used, the heat of the oven must be more cautiously applied. With judicious management, a dense black may be produced in 15 or 20 minutes, though, some times the articles to be japanned are allowed to remain in the oven several hours. When plates are to be japanned, they should be carefully placed in a horizontal position. If the heat is applied too strong at first, it will occasion wrinkles in the surface; and if the work is over heated in finishing, the black will be charred and not adhere. A little experience will satisfy a practitioner on these points.