[The Editor of the Home Laboratory will be gJad to receive any suggestions for this department and w ill pay for them, promptly, if available.] THE etching of metals by electrical energy does not seem to have received the attention it deserves. It must be borne in mind that wherever any chemical body is employed to perform etching, there also is electrical action. Many times when etClhing by electricity has been attempted, the protective top bearing the inscription has been eaten away before any appreciable depth has been attained. This has been caused by the employment of an unsuitable etching fluid, or the protective top was not sufficiently resistant to the separated element or ion liberated by the electric energy. By a little study of the chemical bodies employed for etching either copper, zinc or steel, such as nitric add, perchloride of iron, sulphate of zinc, and chromic acid, it has been found that these bodies are not the best suited for electrical etching. The real point to be considered is the effect of the liberated element upon the metal. Several solutions have been found that will answer the purpose well, and it is safe to predict that in due time the etching of metals by electric energy will become universal, because by the employment of electrIcity, it can be as readily ascertained how far the etching has proceeded, by noting the amperage and the time elapsed, or, in other words, quantity of electricity employed. This, according to Faraday's law, is proportional to the quantity of metal dissolved. By the electrolytic method one man could perform all the etching of six plates in the same time that it takes to etch one by the usual processes. For the protective top, any of the preparations may be used that are employed at present for ordinary etching, such as albumen, fish glue, enamel or bitumen. For the information of those who are not acquainted with the above preparations it may be stated that the following will give results that can be depended upon, bearing in mind that where water is mentioned, distilled water is meant at all times. SENSITIZING SOLUTIO: FOR ZINC. The white of one fresh egg (albumen). Distilled water ................... 6 ounces Bichromate of ammonia .......... 20 grains Dissolve the bichromate of ammonia in two ounces of the distilled water. The remaining four ounces of water must be added to the albumen, which must now be well churned into a frothy mass for two or three minutes. Add two drops of strong aqueous ammonia to the egg .mixture, ·using the egg beater, then add the bichromate mixture, beat again well, add half an ounce of pure photographic akohol, using the egg beater again, so as to insure perfect incorporation. After the bichromate has been added to the albumen, the mixture becomes sensitive to light, and therefore all subsequent steps should be carried out in red light, or at any rate in subdued daylight. Allow the mixture to stand over night, then filter it through a tuft of absorbent cotton pressed lightly into the neck of a clean glass funnel, placing a flat strip of glass into a wide mouth bottle diagonally under the tip of the funnel, so that the sensitive albumen mixture falls drop by drop upon the glass strip. The object of this is to prevent the forming of air bubbles. The filtering must be conducted undtr a deep orange colored light. If copper or steel is to ,be employed, the following mixture must be made: SENSITIZING SOLUTION FOR COPPER. Photo-engraver's fish glue ...... 211' ounces Albumen (fresh) .............. % oun(e Bichromate O'f ammonia ........50 grains Dissolve the bichromate separately, add this to the fsh glue and albumen. The mixture must now be well churned by a rotary egg whisk or beater, then fltered as described for the zinc sensitizer, but preferably twice. The etching solution must be made up, and kept in a glass battery jar. The etching solution given here is a special mixture for electricaZ etching only. ETCHING SOLU1ION. A saturated solution of common saU, about 3 pints Hydrochloric acid ...........2 fluid drachms Water ......................5 ounces This solution will keep well. The use of hydrochloric acid is to correct for the liberated sodium hydroxide which is formed by electrolysis; the libec-ated chlorine performs the etching, and combines with the zinc or copper, forming chloride of zinc or chloride of ,copper. In the case of steel, perchloride 'f iron is formed. Each metal must be etched in a separate solution. It will ;be 'Observed that, after etching, the solution for zinc will remain white, that for copper will turn green, and the solution for steel will become a brilliant yellow. To perform the etca-.ing, an ordinary half-tone negative (reversed) must be employed, or .for line etching an ordinary dense line negative, made by the wet collodion process. Procure a few pieces of zinc plate about one-sixteenth of an inch thick, the same as used by photo-engravers. Carefully clean the plate by wiping it all over, back and front, with a warm solution of common washing soda. Rinse the plate in clean water, and allow to dry. This operation is simply to rid the surface of any greasy matter introduced by handling. The plate First arc lamp, Sir Humphry Davy, 1808 From an old engraving, 1812. must now be cleaned thoroughly, with fine pumice powder and water, by placing it Lace up upon a clean board, using a clean rag dipped into the pumice powder, and rubbing well up and down, not in circles. The plate must be reversed, so that the bottom end becomes the top, and the rubbing continued until, when washed with water from a faucet, the water will remain free and even all over, showing no greasy streaks or spots. Now drain the plate after it is well washed in running water all over, and pour a small quantity of the sensitized albumen mixture (under an orange light) upon one end of the plate. Halftone electric etchmg on copper. From an old photograph. Tilt it so that the water is driven before the albQ-men, and allowed to run off. Drain the plate; then pour a small quantity of the sensitive mixture upon the opposite end and drain this off, allowing only a small quantity to remain UP0l the plate, which must now be gripped tightly at one comer by a pair uf flat-nose pliers, and held level over the flame of an alcohol lamp, or small gas stove. Blow upon the surface with the breath, so as to aid evaporation of the water. Care must bO taken that the plate does not become too hot, be(ruse this would cause the albumen to become insoluble. The plate now being dry, may be laid upon the negrtive, which has previously been placed in a photographic printing -frame. A piece of felt is placed upon the back of the zinc plate, the back adjusted, and the springs, which should be very stiff, hitched into place. The negative must be allowed to rest upon a stout piece of flat glass in the printing frame. The frame can now be placed in the sunlight. If this is very bright, and the negative is a clear and brilliant one, the exposure will be about one minute and a half to two minutes. The frame must now be returned to a room illuminated by a deep orange colored light, the zinc plate removed, laid back down upon a stout sheet of glass, and a small India rubber roller, which has been carefully rolled up with a small quantity of lithographic ink, is passed over the printed surface 'f the zinc, roIling from front to back. The plate may now be reversed, and rolled until the surface presents a uniform gray coating of ink all over. Now place the inked plate into a tray of clean water, let :t remain for five minutes, then take a tuft of wetted absorbent cotton, and carefully wipe the inked surfaee still under the water, when, if the exposure has been right, a beautiful print in minute dots or lines will present itself upon the surface of the zinc. The plaie must now be removed, washed under the faucet, and carefully dried, then powdered all over with dragon's blood, the exooss being removed by tapping the back with a sharp blow of a hammer handle, holding hammer head in the hand. The surface must now be lightly brushed with a flat camel's hair brush and the plate heated over a gas f'ame, until it begins !O smoke faintly. Then lay it upon a piece of cold metal for about two minutes, when it will be ready for the etohing bath. Two cells of the size and kind described in the Scien1ific American of May 28th, 1910, page 445, will form the battery for generating the requisite electrical energy. The elements must be coupled in series, viz., the carbon of one cell connected by a piece of copper wire to the zinc of the next. A piece of sheet copper, zinc, or brass must be soldered to a copper wire and attached to the zinc cylinder of the free element. This will form the cathode. It must be remembered that electrical etching is the reverse of electroplating. The piece of zinc bearing the ink image must be brushed over every part that is not to be etched, ,with either shellac varnish or a thin coating of asphaltum varnish, which is best done when the plate is warm. This ,protection will 'revent the plate from being attacked, except where the required etching is to take place. The plate must now be attached to the remaining carbon element of the battery, by simply soldering a tin or brass garter clip to the end of a strip of copper wire, the right kind of clip being one of those with saw-like teeth. This will grip the plate perfectly, and allow easy removal. The plate, now being firmly gripped, must be quickly lowered into the salt solution. Instantly gas will be evolved from the cathode, while the metal exposed between the dots will begin to dissolve. After two minutes' aC'tion the plate should be lifted and examined, when in nine eases out of ten it will be found to be satisfactory. Dip the plate into the liquid again for three minutes, then remove it, and pour a gentle stream of clean, cold water over the surface. Now use a biconvex lens of about four-inch focus to examine the surface, -when it will be found that for all ordinary printing purposes, the etching wi1l be deep enough. The albumen top will hold well in this etching solution, because it is well known that albumen becomes coagulated in a strong solution of common salt. If upon examination it is found that the etching is not deep enough, the plate should be very carefully brushed all over the surface with a flat camel's hall brush; then washed, dried and the ink roller carefully passed over the surface again; then warmed to partly set the ink, powdered again with dragon's blood, heated to melt this again, cooled off, and etched again for two or three minutes. As a rule, this second etching is not required. The surface must not be rubbed with absorbent cotton, because the edges of the etch are very sharp, and the fiber of the cotton will adhere very tenaciously. The plate may now be oleaned with either wood alcohol and ammonia, eor if asphaltum has been used, ben2ine or turpentine must be employed to remove it. When the plate is cleaned, it can be trimmed at the edges, and pinned to a wooden block to make it type high, ftted in with type in the usual way, inked up and printed. If the etching is to be done upon copper, the fsn glue enamel must be used, because it is better suited for copper than the albumen resist. Procure a piece of sheet copper the same as used by photo-engravers, that has been faced, polished and buffed, Clean it in the same way as describe! for zinc. Use (Continued o page 177.)
This article was originally published with the title "Etching Metals by Electricity"