Etching Solutions - acid, solution and water

ETCHING SOLUTIONS For zinc etching nitric acid is invariably used, the strength varying from I to 20 per cent. according to the stage of the etching and the nature of the work. The bath has to be constantly rocked whilst the plate is being etched. A " still " etching solution, which does not require rocking, consists of :— Nitric acid . . . 13o parts Water . . . . zoo „ Sal ammoniac . . . 20 „ Pyroligneous acid (wood vinegar) . . . . 20 „The bath should stand two or three weeks after miming. Another bath for zinc which need not be rocked is : Sulphuric acid . . . 6 parts Potassium nitrate . . 2 „ Water . . . . 20 „ Dissolve the potassium nitrate in water, and then gradually add the acid. Dilute with water till bubbling ceases.

For etching an enamel film without " burning in " the image, the following bath is recom mended : Alcohol (4o %) . 400 parts Nitric acid . . . 5 to 7 „ For half-tone copper etching, iron perchloride is dissolved in water until the solution registers from to on the Beaume hydrometer (up to 1•36 sp. gr.). About if lb. perchloride to i pint of water will bring the solution to the required strength. The solution is improved for immediate use by adding f pint of an old bath to every quart of new. Rocking the bath

makes the etching proceed more quickly. Heat also aids the etching. Sometimes the plate is etched face downwards, held in a damp.

Brass is also etched with perchloride of iron at 35° Beaume.

Steel can be etched with ferric perchloride at 4o° Beaume, or with a strong solution of chromic acid, or with acetic acid five parts, fuming nitric acid one part, diluted as may be necessary with distilled water.

The term " etching solution " is also applied to the solution of glycerine, with other ingredi ents, such as liquor ammonia, calcium nitrate, sodium chloride, etc., used for damping the collotype plate. Again, " etching " is applied to the operation of spreading over the litho graphic stone or zinc or aluminium plate a slightly acid gum solution, which prepares the surface for dean inking and printing, though it does not actually etch into perceptible relief. Such etching solutions for zinc contain a decoc tion of nutgalls and phosphoric acid, and for aluminium phosphoric or hydrofluosilicic and other acids.