Although many hard solders contain copper, and might therefore be classed with copper solders, we will here consider under that name only those whose essential constituent is copper. Copper is a metal which melts only at very high tem-peratures, and affects in this way its alloys with other metals; any solders containing copper are hence always to be called hard. But the fusing point is generally lowered as the amount of copper is decreased.

Pure copper, on account of its strength and tenacity, is an excellent material for soldering, and is much used for cast iron, wrought iron, and steel. Where its color is no objection, it is to be highly recommended for use with the above-mentioned metals. It may be employed in the form of thin strips, or in filings scattered over the place to be soldered, this according to the nature of the surface.

Fine copper filings are sometimes used to solder copper itself, and with the best results; but usually alloys containing a large percentage of copper, but more easily fusible than copper itself, are used.

These (used also for bronze) are mixtures of copper and lead. The more lead they contain the more readily fusible they become, of course, and the less they resemble copper in color or in point of tenacity. The most common copper solder is composed of 5 parts of copper to 1 of lead. Another has copper 80 parts, lead 15, and tin 5.

Copper amalgam, that is, a compound of copper and mercury, is very well adapted to soldering such copper and bronze articles as cannot be exposed to strong heat. This is prepared by first precipitating the copper from a solution of blue vitriol, which is done by putting in sheets of zinc and shaking. The copper will be in the form of a very fine powder. From 20 to 36 parts by weight of this powder, according as the solder is to be harder or softer, are put into a porcelain mortar, and enough sulphuric acid is poured on to make a paste, then 70 parts of mercury are stirred in. After a uniform mass has been obtained, the sulphuric acid is washed out, and the amalgam will be left, after ten or twelve hours, as a hard mass, capable of being polished. If heated to 662 deg. F. it becomes soft and malleable.

In soldering, the seams of the copper or bronze articles are brushed over with, a solution of mercury, the so-called amalgamating fluid, and become white from the separation of mercury. The amalgam, powdered, is scattered over, and by passing the hot soldering iron over the places, the soldering is completed. The amalgamating fluid is made by dissolving 10 parts by weight of mercury in 11 parts of nitric acid, and diluting the solution with 500 to 550 parts of soft water.