Etching copper is not a very difficult process. Secure a sheet of No. 16 gauge copper of the width and length wanted for the back of the thermometer. In the design shown the extreme width is 3-1/2 in. and the extreme length 7 in.
Illustration: Copper Thermometer Holder
Draw a design. The one shown is merely suggestive. The worker may change the outline or proportions as desired. The decoration, too, may be changed. The essential thing is to keep a space upon which to place the thermometer. This design is in what is known as two-part symmetry. A line is drawn down the paper and one-half of the outline and decoration worked out. This done, the paper is folded along the center line, a piece of carbon paper is inserted between the folds and the design transferred on the inner surfaces by tracing with a pencil over the half of the outline previously drawn. Trace the design and outline upon the metal, using the carbon paper.
Cut out the outline with metal shears and file the edges smooth.
With a small brush and ordinary asphaltum or black varnish, paint the design, the margin and the entire back of the metal. When this coat has dried put on a second and then a third. The asphaltum is to keep the acid into which the metal is to be immersed later from eating any part of the metal but the background. Two coats or more are needed to withstand the action of the acid.
The acid bath is composed of nitric acid and water, about half and half, or, possibly, a little less acid than water, the mixture being made by pouring the acid into the water, not the water into the acid, which is dangerous. Keep this solution off the hands and clothes, and do not inhale the fumes.
Put the asphalt-coated metal in the bath and allow it to remain for four or five hours, depending upon the thickness of the metal and the strength of the acid. With a stick, or a pair of old tongs, take the metal out of the acid occasionally and examine it to see how deep the acid has eaten it--1/32 in. is about right for the No. 16 gauge.
When etched to the desired depth, remove the piece and with an old knife' scrape off the asphaltum. Finish the cleaning by scrubbing with turpentine and a brush having stiff bristles.
If the metal is first covered with turpentine and then heated over a flame, all the colors of the rainbow will appear on its surface. These colors fade away in the course of a long time, but they can be easily revived. Another way to get these colors is to heat the metal and then plunge it into the acid bath quickly.
A green finish is obtained by painting the background with an acid stain composed as follows: 1 part ammonia muriate; 3 parts ammonia carbonate; 24 parts water. If one coat does not give the depth of color desired, repeat as many times as is necessary, allowing each coat time to dry before applying the next.
To "fix" this color so that it will not rub off, and to keep the metal from tarnishing, apply a coat of banana oil or lacquer.
Thermometers of suitable size can be bought in either brass or nickel. They have holes through their top and bottom ends through which metal paper fasteners can be inserted, and these in turn put through holes punched in the copper back.