Upright Butt Seam.
The upright butt seam is seldom used on large work, as it is a difficult matter to make an upright butt seam that will stand the test, as, if a finished seam is cut into short pieces, an examination of the severed ends will show many weak places that were previously thought to be very strong. The reason of this is that the heat necessary to fuse through the lead will cause the lead to run from the seam and leave a hole.
The sheets for practice are prepared as described for flat butt seams, and must be securely tacked to a board which can be supported in an upright position. The burning is begun at the bottom of the seam. The flame must be shortened considerably, as the fusing must take place somewhat slower than in flat seams, as in upright or inverted seams the attraction of gravity remains to be overcome, and the operator must have plenty of time between the commencement of brightening and the actual fusing to drive the melting drop to the exact position desired.
Fig. 25.—Burning an Upright Butt Seam.
The blow pipe is held so that the flame strikes the seam squarely and at about a right angle with the sheet, as shown in Fig. 25. When fusion starts the flame should be drawn quickly to one side, and if the lead is at the proper temperature the melted drop will follow the point of flame, and as it comes in contact with the adjoining edge it will properly unite. It is not necessary to add lead to these seams oftener than at intervals of 5 or 6 inches, or as often as the lead shows signs of weakening, when it may be added by holding the lead strip against the lead sheet and slightly above the flame. The melted drop will unite with the sheet and can then be driven to any desired position. This seam will show the characteristic beads, but they will lie nearly level with the lead sheets, and if a scratch cloth be rubbed over the seam all traces of the position of the seam will be removed.
To make a really strong seam it must be gone over with the flame at least twice, as after fusion of the edges takes place the flame can be used quite strong without fear of the lead running from the seam. Do not leave this seam until you are satisfied that it is nearly perfect. It is good practice, and every hour spent only makes the mastery of the next seam come so much more quickly.