Glue, Shellac or Oil? Property Tests in a Violin Shop

Four Materials at the Repair Bench

1Every Saturday, seventh grader Rafael Quintero helps at a violin repair shop run by Ms. Ilse Varga, a luthier, or maker of stringed instruments. A violin is held together with glue and protected by thin coats of finish. Before Ms. Varga repairs an old instrument, she likes to know what each material is and whether it formed a new substance when it hardened. A material that only dried or cooled can often be softened again with the right liquid or a little warmth, which makes a repair much easier.

2Rafael followed four materials through the shop's week. Hide glue starts as dry amber granules made from animal skins. Ms. Varga soaks them in water, then warms the jar in a pan of hot water until the glue flows like honey. Brushed onto a joint, it cools into a stiff gel and dries hard over about a day.

3Shellac starts as thin orange flakes. Ms. Varga dissolves them in alcohol in a closed jar. When she brushes the liquid onto wood, the alcohol evaporates within minutes and leaves a thin, shiny film. Rosin, which players rub on bow hair, comes in small cakes. A customer dropped one, so Ms. Varga warmed the broken pieces in a double boiler until they flowed, and Rafael watched her pour them into a round mold, where they cooled into a new cake.

4Raw linseed oil is a golden liquid pressed from flax seeds, with a strong, nutty smell. Ms. Varga rubbed a thin coat onto a scrap of maple and left it in a warm, airy room. For several days the coat stayed sticky. After about a week, it had become a dry, hard film with almost no smell. She also coated a glass plate several times so that Rafael could peel off a piece of the hardened film to test.

5Rafael tested a sample of each material before and after its change. For solids, he cut neat pieces, found their volume from their length, width and thickness, and divided mass by volume to find density. For the oil, he weighed a measured volume. Ms. Varga handled the alcohol and mineral spirits for the solubility tests and used a melting-point tester. The results are in Table 1.