In practice there are the usual number of details requiring skill that the smith must observe.

The contact faces should always be shaped so that their middle points touch first and so that hammering causes union first at the middle. This prevents slag becoming enclosed.

The heating should be done rather slowly, so that the pieces will be of uniform heat at the weld. Both irons should be of the same temperature. To effect this the smith must not blow up his fire too hot and must watch the color of both irons. A deep bed of coals will give a flame the least oxidizing, because most of the oxygen will burn to carbon monoxid or dioxid.

The welding heat is quite different for different irons. The smith must use his judgment, taking good care he does not overheat his irons. Overheated iron, miscalled "burnt iron," will be brittle on cooling. Steel is much more sensitive to overheating than iron. It is seldom safe to heat steel above redness.

The iron should be well fluxed directly on the contact surfaces, and not merely around them, as is too often the case. The common fluxes for iron are pure silica (river sand) and borax. Impure malleable irons are self-fluxing to a certain degree, but this cannot be relied on. Other fluxes are calcined borax and sal ammoniac. One writer1 recommends powdered marble, which is limestone, the flux of the blast furnace. Borax is recommended for steel on steel or steel on iron, as silica is inadequate.2 Besides fluxing the bars before raising them to welding heat, the smith may sprinkle calcined borax on the irons when they are ready to weld, to replace it on surfaces accidentally rubbed bare in the forge.

1"The Blacksmith's Guide," J. S. Sallows.

2 "Testing of Materials of Construction," W. C. Unwin, 1899, p. 292.

The hammering, or "working," is very important, and must be done rapidly. The smith manipulates his bars so that welding begins at a middle point and works outward, driving the slag away from it. The first few blows of the hammer should not be heavy. If the pieces are large or the smith slow, his heat will fall before the weld is finished and he must put his bars back in the fire. Second heating must be avoided if possible; slag is apt to get in the weld or the pieces may become "burnt," to say nothing of the time lost.

Fig. 3. - Lap weld. No upset.

Fig. 4. - Lap weld. Upset.

The different kinds of welds known to the smith - butt, lap, scarf, jump, cleft - are variations of simple welds. Illustrations of them and of how the-stock should be shaped are given in figures 3 to 7. The method of hammering will suggest itself in each case. Jump and butt welds will have sufficient upset to work on, while scarf welds will not unless the pieces are jumped or over-lapped considerably.