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TABLE OF CONTENTS

THE USE OF ACIDS IN ETCHING

MATERIALS

One pound chemically pure nitric acid. Kitchen measuring glass. Glass tray. Package of pipe cleaners. Large white blotters. Bottle of turpentine. Glass or picistic funnel. Box of bicarbonate of soda. Bag of powdered pigment (black).

BITING THE PLATE

Before attempting any biting of a picture on a plate it is advisable to make a test plate.

The Bath

Most biting is done in an acid bath. This process involves imrnersing the plate in a tray of acid (the bath) and allowing the acid to etch, or bite, the lines to varying depths through different periods of immersion.
The most commonly used etch is nitric acid. Other acids will be discussed later in the book. Chemically pure nitric acid is a powerful mordant. In its pure state it is so corrosive in its action on either zinc or copper that it would quickly destroy both the ground and the plate. It must be reduced by adding water to a workable and controllable formula. The proportions of water and acid vary, depending on many factors. For work on zinc or on copper different formulas must be used. Also the artist's individual approach to etching affects the mixture of acid and water he employs. The following are basic beginning formulas. After some experience with etching, the artist can vary the formulas to his personal taste, increasing the proportion of acid for a swifter etch or decreasing it for a slower etch. For zinc plates: One part c.p. nitric acid to nine parts cold water . For copper plates: Three parts c.p. nitric acid to five parts water Many factors affect the action of the acid on the plate. Only with experience can the etcher learn to control these factors. The temperature of the room affects the temperature of the bath. The warmer the bath, the faster the bite. The depth of the bath affects the speed of biting. A shallow bath will etch slower than a deep bath. Exposing the tray of acid to the air gradually weakens the strength of the acid bath. Since all these elements are not measurable, prescribed formulas cannot be given for dealing with every situation.

MIXING THE ACID

Any glass container can be used, but an ordinary kitchen measuring glass with a lip will be of he,ip in pouring. Use an empty, glass-stoppered acid bottle (a stopper made of anything but glass will be cor roded by the acid fumes) to hold the mixed formula. Use a glass or acidproof plastic funnel. Mix enough of the etch so that the acid when poured into the tray will be deep enough to cover the plate with at least 1/4 inch of solution. Pour the desired amount of acid carefully into the measuring glass. Then, using the glass funnel, pour the acid into the bottle. Measure out the water needed for the correct formula and pour this into the bottle with the acid. Paste a label on the bottle and mark the formula on the label. The etch can be used several times, but it grows weaker in action with each use. Some beat is generated in the bottle when the acid and water are mixed. Allow the etch to cool before using. Warning! Important! Nitric acid should be handled with great care. It is highly corrosive and can burn the skin severely. It is particularly dangerous to the eyes.

HANDLING THE ACID

Bottles with acid should be stored away from other etching supplies, particularly etching plates. The fumes can corrode the plates. When opening the bottle, keep your face away from the mouth of the bottle to avoid the acid fumes. When working over etching tray, keep alert to avoid any accidental splashing caused by the plate slipping into the bath. Keep a box of bicarbonate of soda, or baking soda, at hand. Should acid splash on your skin or in your eyes, act fast. First let cold running water flood the area for a few minutes. If acid does get into the eyes it is necessary to force them open. Then apply a paste of soda and water. This stops the corrosive action of the acid. Consult a doctor immediately.
Any deep glass or plastic tray can function as an acid tray. The glass trays from iceboxes or photographers' developing trays are excellent. Pour the etch into the tray. Be sure that there is enough etch to cover the plate, when it is immersed, with at least 1/4 inch of the solution. To get the plate into the bath without splashing, either of two methods can be used. Put one end of the plate in the acid with the edge of the plate against the side of the tray so that the plate cannot slip. Slide a pencil under the opposite end of the plate. Using the pencil as a lever, slowly lower the plate into the bath. (The plate can also be lifted out of the bath with the pencil.) Or lay a doubled piece of string under the plate. lower the plate slowly into the both by rnanip- ulating the ends of the string. The string remains in the bath, under the plate, until the biting is finished. Then the plate is lifted out by the string. Working conditions should be kept as nearly constant as possible. Room temperature affects the speed of the biting. The acid etches faster in a warm room than in a cold one. Some etchers use a ther- rnometer to test the temperature of the bath. This is not essential. With experience one can learn to gauge quite accurately the action of the acid and to allow for temperature variations. After all the biting is completed, the ground must be removed from the plate. Pour turpentine over the plate and remove the ground, using a soft cloth. Remove the stop-out from the back of the plate. If lacquer is used as a stop-out, alcohol or paint remover will take it off.

TESTING THE BITE

Often the etcher wants to see how the plate has bitten so that he can tell whether he wants to do more work on it before printing. A simple way to do this is to pour a little powdered black color onto the face of the plate. Using the fingers, rub the powdered color over the plate. This forces the black powder into the lines. Shake off the surplus powder. The black-filled lines, contrasting with the light gray of the plate, give a fairly exact image of the finished print. The black pigrnent can be washed out of the plate with cold water. If the plate seems satisfactorily bitten, then the next step is printing.

THE TEST PLATE

Because biting time plays so important a part in etching, it is advisable for the beginner to do a test plate before attempting a serious one. This will help him determine the biting time necessary to obtain variation in the blackness of his line. On the test plate he can practice various types of line work used in etching and study the way different periods of immersion in the acid bath vary the depth of the line and affect its lightness or darkness in printing. Ground a small plate with rubbing ink. Copy the line work shown on the test-plate diagrams. Notice that the top set of lines consists of single strokes, the second set is crosshatched twice, and so forth. When this is done, cover the back of the plate with stop-out varnish to protect it from the acid. The plate is then ready for the acid bath.

TIMING THE BITE

Immerse the test plate in the bath. Watch the lines on the plate for bubbles, which form because of chemical reaction when the acid begins to etch the metal. Wait until bubbles have formed on all the lines - the single lines will bite last - and then begin timing the bite. During the entire biting time the bubbles must be brushed off the plate frequently, about every 15 seconds, using a feather or a pipe cleaner. If the bubbles are permitted to rest on the lines they block off the acid, and the line will bite irregularly. The first biting time is 60 seconds after bubbles have formed on all the lines. Remove the plate from the bath. Blot all acid off the plate, then brush stop-out varnish over the section of the plate which calls for the shortest timing. Wait until the stop-out is dry; then immerse the plate for the second bite. Repeat this process until the whole plate has been bitten.
In timing the bite, always deduct the previous biting time from each timing period. The first bite is 60 seconds. The second bite is 5 minutes less 60 seconds. The third bite is 20 minutes less the previ- ous 5 minutes, etc. After the lost bite, clean the ground off the face and the stop-out off the back of the plate. Bevel the edges of the plate with a file before printing.

Referenced from Modern Methods and Materials of Etching By Harry Sternberg