Horizontal Butt Seam.
This form of seam cannot be used to any advantage on general work, but, like seams in other positions, it cannot always be avoided. The practice sheets are prepared and tacked securely to the board, as previously described, and are then placed in the position shown in Fig. 26. The position of the blow pipe is as shown at a. The flame should strike the sheet nearly square. The edge of the upper sheet should be heated first, and as it brightens the flame should be directed onto the edge of the lower sheet. If properly done, fusion will at once take place.
Fig. 26.—Burning a Horizontal Butt Seam.
The object sought is to get a light fusion between the two sheets before attempting to burn the lead clear through the seam. If this is not done, the lead will run from the upper sheet and cause holes, or at least will seriously weaken the upper sheet, as shown in cross section at b. After fusion is once obtained it is a simple matter to go over the seam a second time, which can then be fused clear through without much danger of burning holes through the sheet. Lead can be added in the same manner as in upright seams if necessary.