Inverted Butt Seam.
This seam is used extensively in joining waste pipes which conduct the acid from tanks to the drain. These pipes are usually in a horizontal position and the seams must be burned in place. The most difficult part of the seam is in starting it. When fusion has once taken place the balance of the seam is easy. The seam is prepared the same as described for other butt seams. Care must be taken to have the edges butt close. The board can then be supported in the required position by any convenient device.
The blow pipe flame must be made as short as possible and still melt the lead. The point of the inner flame is then placed squarely on the seam. Both edges must be heated at once. If the edges begin to brighten and do not show an inclination to fuse, the flame should be drawn quickly to one side, and the melted drop will follow the point of flame and unite with the adjoining edge. This seam, in common with the other butt seams, should be gone over the second time to assure a perfect seam. It is difficult work to add lead to the flat inverted seam. When necessary to do so, however, it can be added by burning the end of the lead strip to the seam. The strip is then melted off, leaving a drop of lead affixed to the seam, which can then be drawn to the required spot with the flame.
The characteristic inverted seam shows pits upon examination of the reverse side of the sheets. These are caused by overheating. The operator will often be surprised at the inverted butt seam showing a remarkable fullness. This is accounted for upon the examination above referred to. The lead, upon the application of the heat, runs from the upper or back side of the sheet and forms a very full seam. For that reason the inverted butt seams always appear stronger than they really are. See inverted lap seam, Fig. 27.