When scraping the tin bright, do not scrape it so hard that all the tin will be scraped away from the inner sheet of iron, as solder will stick to tin much better than to iron. If the tin is not very dirty, a piece of emery cloth or sand paper may be used to clean the joint.
Paint cans, cans that have contained stove blacking, rubber cement, varnish, shellac, etc., should be thoroughly boiled in a strong lye bath before soldering; paint is usually made of oxides and oxides are a sure preventive of soldering. The lye bath is made by adding two heaping tablespoonfuls of lye or washing soda to the gallon of boiling water. Cans boiled in this solution for five minutes will be thoroughly cleaned and free from paint, paper labels, and practically anything likely to be found inside or outside of a can. The lye or washing soda may be obtained at any grocery store. Care should be taken not to get any of the lye solution on the hands or the clothes as it is very caustic and will burn the hands and ruin the clothes if not immediately washed off. The work should be handled with a wire hook while in the bath and well rinsed with water when removed from it. The same lye bath is used before paint is applied to tin work, when all forming, soldering, riveting, etc., is done. It removes the flux, acid, and finger marks, leaving a clean surface on which to paint.
Soldering a Practice Piece.—For practice in soldering, an angle joint is a good thing to begin with; something that is small and easily held in position while being soldered. As I have already described the forming up of a biscuit cutter to the point of soldering it together, a practice piece resembling it will be an excellent thing with which to begin.
Cut a narrow strip of tin about 1 inch wide and 4 inches long and a flat piece of tin about 2 by 3 inches. Be sure that the ends of the narrow strip are cut squarely across, using the square if necessary. (See [chapter on “Laying Out Work,” page 32].) See to it that both pieces are well flattened out and smooth.
Bend the narrow strip into a semi-circular form, like the biscuit cutter you have already to solder and stand this piece in position on the larger flat piece of tin.
Now lay the piece near the soldering copper heater, on the wooden bench; be sure to place it on wood, not on a part of the vise or any other metal that may be convenient. Iron, stone or brick will absorb too much heat from the tin if directly under and in contact with it, and thus prevent soldering.
Apply a small amount of soldering paste to each joint as indicated in [Fig. 18]. The paste may be applied with a small flat stick of wood such as a match stick shaved down to a long, thin, wedge point.
Fig. 18.