Etching the stone

The following factors govern the selection, application and effectiveness of etches during the process of desensitizing the stone:

1. The grease content of the drawing

Drawings week in grease content require relatively little acid to liberate their fatty components into the stone. Excessive acid in the etch will destroy weak grease deposits and will attack the grain of the stone, coarsening The image. Drawings containing heavier grease deposits require proportionately greater amounts of acid to liberate their contents.

2. The volume and PH of the etch

Formulas for etching small stones up to 55x30 cm are based upon a single ounce (30 ml) of gum arabic. Larger stones require a minimum of two ounces (60 ml) of gum arabic in the etch solution to cover their surface effectively. Customarily, formulas for etching stone specify a given number of drops of acid per 30 ml of gum arabic, The relative acidity of the solution is referred to as its pH value. Increases in either the number of drops per 30 ml or the total volume of the etch solution will significantly alter its pH value through logarithmic progression.

3. The cumulative result of etching

Theoretically, there is an ideal point at which the strenght of etches and the cumulative result of their application will totally desensitize any given drawing without injuring either its image or its nonimage areas. In a few cases the single application of a week etch issufficient to desensitize a delicate drawing having meager fatty content. Most drawings, however, require a total desensitization that cannot be achieved by a single application.
While one of sufficient strenght would weaken or destoy image formations, a single application of of a weaker solution would be insufficient to desensitize the nonimage areas. Therefore, the printer must work out practical approaches to ensure that the cumulative effect of etching will impair neither image nor nonimage areas. The standard procedure is to etch the work in two separate stages. The strenght of the first etch should be sufficient to establish the drawing deposits so that they may be replaced with ink. The second etch completes the desensitization by fortifying both image and nonimage deposits. Several etches of different strenghts may be required for drawings in which the grease content varies greatly over an extend range.

4. The hardness of the stone

Hard and dense stone may be etched with relatively strong solutions. Soft stones may be etched with weaker solutions; otherwise their grain will be coarsened or destroyed. Thus drawings that require stronger etches should be placed on hard stones, or, if its not possible, etching should be carefully controlled to achieve total desensitization without eroding the stone’s surface.

5. Atmospheric conditions

The chemical action of an etching solution is accelerated by high temperatures and dry atmosphere; it is retarded by low temperatures and humid atmosphere. If, due to high temperature and humidity, the etch reacts too quickly, it is difficult to control its application. For this reason, weaker etches should be used in summer than in winter in regions having variable climates.

6. Reaction time for etch solutions

When the etching solution makes contact with the stone, its chemical reaction will vary in time from about a half minute to three minutes, depending on the strenght of the solution, the hardness of the stone, the grease content of the drawing, and the atmospheric conditions. Present evidence suggests that the major reaction occurs during the period of time in which carbonic gas is being produced from the contact of acid with the stone. Gradually the carbonates of the stone neutralize the reaction, after which the solution is spent. Several applications of solution are often necessary to liberate and localize heavy grease deposits, whereas only single applications may be necessary for weaker deposits. Since many types of drawings contain both kinds of work, the manipulation of the etch to favor one area or another becomes a critical matter of control. Some printers control strong etches by making several application, arresting each of these before the reaction is spent; cumutativety they are sufficient to the task. Other printers use weaker solutions applying them. for longer periods of time to allow for more deliberate manipulation. These alternatives are possible because a stronger solution applied for half the time will accomplish much the same result as a weaker solution applied over a longer period. A combination of both approaches is often necessary, in order to achieve effective desensitization.

Etching solutions

Gum arabic is a major constituent of lithographic etches for stone. Although normally water-soluble, it is capable of forming, on the nonimage areas of the stone, adsorbed film that permit the retention of water for prolonged periods. Image deposits are formed in the stone by including varying proportions of nitric, phosphoric, or tannic acids with the gum solution. The acids liberate the fatty components of the drawing materials, premitting them to unite chemically with the stone. In the early days of lithography, the liberation of the drawing components into the pores of stone was accomplished by followinig solutions of nitric acid and water onto the drawing surface; this was followed by a separate operation of gumming to desensitize the nonimage areas. Present-day procedure incorporates the acids with the gum for simultaneous application and preparation of image and nonimage areas.

Etching solutions are prepared by measuring the gum arabic in measure. The most efficient method for meassuring nitric acid is by dispensing from laboratory drop bottles. Acid can also be despensed by pipette, burette, or eye dropper.
Etching solutions are istributed on the stone by brushing them on with special acid resistant etch brushes or applying them with well-saturated sponges or cotton tufts, or flowing them across the stone with well-saturated cloths, which cover the entire surface in one wipe. Distribution with the etch brush is by far the easiest and best method of control. It should be noted that application by cloth or cotton swab will usually require a greater volume of etch in order to saturate the carrying instrument fully and still permit coverage of the stone.

The etch table
Weak etch
30 ml gum arabic
+ 6 - 12 drops of nitric acid
Moderate etch
30 ml gum arabic
+ 13 - 18 drops of nitric acid
Moderately strong etch
30 ml gum arabic
+ 19 - 26 drops of nitric acid
Very strong etch
30 ml gum arabic
+ 27 - 33 drops of nitric acid
drops of acid
/ 30 ml of
gum arabic
PH
1 drop
3,8 - 3,4 PH
2 drops
3,5 - 3,2 PH
3 drops
3,4 - 3,0 PH
4 drops
3,1 - 2,8 PH
5 drops
2,9 - 2,5 PH
6 drops
2,7 - 2,3 PH
7 drops
2,5 - 2,0 PH
8 kropel
2,3 - 1,8 PH
9 drops
2,0 - 1,6 PH
10 drops
1,8 - 1,4 PH
11 drops
1,7 - 1,3 PH
12 drops
1,5 - 1,2 PH
13 kropel
1,5 - 1,1 PH
14 kropel
1,4 - 0,9 PH
15 drops
1,3 - 0,9 PH
16 drops
1,1 - 0,8 PH
17 kropel
1,0 - 0,8 PH
18 kropel
0,9 - 0,7 PH
19 drops
0,9 - 0,6 PH
20 drops
0,9 - 0,6 PH
21 drops
0,8 - 0,5 PH
22 drops
0,7 - 0,5 PH
23 drops
0,6 - 0,5 PH