A technical dictionary of printmaking, André Béguin.

Back to Main Page of Technical Dictionary of Printmaking
Attacking agents.

outline of the article:

  1. ACIDS, nitric acid, sulphuric acid, hydrochloric acid, phosphoric acid.
  2. ALKALIS, soda, potash, ammonia.
  3. SALTS, salts of acids (in particular iron perchloride).
Under this heading the reader will find all of the products used in print making to bite, corrode, and scour cloth, paper, stones, and, of course, metals.

A . NITRIC ACID. Nitric acid is a strong acid. Known as aqua fortis to the ancient alchemists, it is the most often used acid in etching. The term aqua fortis and its prevalent use in etching account for the fact that the term for etchin was derived from it in many languages, as in French (eau-forte), Italian (aquaforte), etc. Nitric acid can also be used for scouring plates and preparing lithographic stones. When pure it is a colourless liquid. It can be bought at various concentrations, such as 36°B, in which case it contains 700 grams of pure acid, or at 40°B (usually called pure) which means that there are 875 grams of pure acid per liter. Nitric acid is characterized by a direct, rapid, and nervous biting which, however, is only superficial and widespread although strong. It bites all metals, including lead and silver, except for aluminium plates and most stainless steels. It will also bite all organic matter. This acid is very sensitive to temperature changes (never leave it in the sun). The following substances resist its biting: rubber, gutta percha, tar; asphaltum, wax, resins, paraffin, glass, ceramics, and various types of plastic. When in contact with a metal it produces bubbles of nitric oxyde which tend to protect the plate from further biting. Nitric acid should be kept in a jar or carboy made of glass or stoneware closed with a ground glass stopper (the inside of the stopper may be slightly oiled) or with a special plastic plug. It must be remembered that this acid is light sensitive and can be ruined by to much exposure to light. Because of this it should be kept in a dark place. The vapours of nitric acid are very toxic but are not dangerous if not inhaled. Avoid inhaling the fumes dispersed above the biting pan. A proper and continous airing of the working area should, however, be arranged. In case of skin burns wash the area with a lot of water and then with cold prepared sodium carbonate. One can also put a plaster of white magnesium on the burnt area.
The concentration of nitric acid should be lowered to about 25° when biting copper plates. If the mixture must be prepared without an hydrometer the following recipes will do. For a strong mordant mix 1/3 acid and 2/3 water while if you need a weaker mordant use 1/4 acid and 3/4 water. For zinc and steel biting the mordant should be between 5° and 15°B. You will obtain a 5° mixture by using one part acid and nine parts water. When this acid is used for scouring* it must be even weaker: mix a solution of 50ml acid for every 1000ml of water into some alum.
When nitric acid is mixed with hydrochloric acid the solution will bite gold and platinum [* aquatint, etching, and lithoqraphy.].

B. SULFURIC ACID. In the past this acid was know as vitriol oil and today it is also known under the name of vitriol (see below under the heading salt). Sulfuric acid is strong, liquid, and colourlesswhen pure. It can be had at various strengths, namely 53°B (1054 grams of acid per liter), 61-62°B (1415 grams/liter) and at 66°B (1760 grams/liter). The strongest of these concentrations is the one most suited for technical use. This acid bites violently all organic matter and all the metals usually used in print making except for copper and lead. It is even more dangerous to use than nitric acid. If water is poured into this acid it will splatter and heat-up. Always pour the acid into water and never water into the acid. If burns have been caused do not wash them with running watet under a faucet. It is better to plunge the burnt areas into a large quantity of water, dry the burns, and wash them again with ammonia water or cover the affected areas with cold prepared sodium bicarbonate plasters.
Sulfuric acid used to bite steel with a low carbon content should be mixed in the following proportions: one part acid and two parts water. The biting of such a solution will be quite rapid but also rougher than with nitric acid. The result will be that the lines etched will be deep and wide.
Sulfuric acid is also used to remove steelfacing from copper plates because it attacks steel and does not react with the copper below it. Finally, it is also used to clean the glass surfaces of plates used for photo-engraving.

C. HYDROCHLORIC ACID. Hydrochloric acid is a strong acid which is colourless when pure (at 22°B it contains 300 grams of acid per liter and is considered pure). It is usually used at 20°B in which case it is yellow coloured. It is highly corrosive and is easily water soluble. Despite its corrosiveness it does not burn one's skin like the preceeding two acids (nitric and sulfuric acid) unless it is left to burn for a protracted amount of time. As a consequence it is sometimes used, in diluted form, to clean very dirty hands (but be sure to rince your hands thoroughly after such use). A large amount of hydrochloric acid vapours in accumulation are dangerous as they may cause an explosion.
At times this acid is used in zinc and steel etching in which case it is mixed with four or five times its volume of water. In Germany during the war it was used in order to avoid using nitric acid which was used for military purposes. Hydrochloric acid will also etch stainless steel plates very well when mixed with once or twice its volume of water (or less if necessary). If used to scour a plate it will be diluted with ten times its weight of water.

D. PHOSPHORIC ACID. Commercially this acid, sometimes called orthophosphoric acid, can be obtained in a liquid form which is both colourless and odorless. Three concentrations are usually available: 37,5°B, 60°B, and 45°B. The 45°B solution is usually the one chosen to work with. It contains 65% pure acid and is highly soluble in water. It is a good acid for scouring ferrous metals but is above all used as one of the components of the zinc and aluminium preparations for both lithographic methods and in offset work [* lithography, offset ].

E . HYDROFLUORIC ACID. A strong acid which in its commercially available form is a watery solution of a gas having the same name. It usually contains 70% pure acid and is colourless. Highly corrosive, it will bite all metals as well as glass and ceramics. It is kept in a gutta-percha or plastic container. Usually it is used, in diluted form, to etch on glass and ceramics as well as to scour cast iron.

F . CHROMIC ACID. To be quite precise this acid is chromic anhydride , a water soluble crystal which is obtainable in the form of dark red needles. These crystals are highly soluble, very caustic, and act as oxydising agents. The acid made from these crystals attacks any organic matter. When in its solid form (crystals) it can cause auto-combustion phenomenae when put in contact with paper. It is usually a component in litho and offset solutions.

C. WEAK ACIDS. A certain number of weak acids are used directly or indirectly in print making. Acetic acid is found in two different concentrations both of which are liquid compositions: 8°B which is a 40% concentration and in a more concentrated form which varies between 98 and 100%. This acid is very much water soluble and will freeze below 17°C (about 62° Fahrenheit). Acetic acid is a caustic which will bite acurately and deeply. In the past it was used, mixed with nitric acid, to bite zinc and steel plates. However, this mixture is a dangerous one as it is liable to explode. Acetic acids, at present, are most often used, mixed with other elements, in offset and litho work.
Another weak acid is citric acid which comes in the form of a mass of colourless crystals tasting of lemon. It too is very soluble in water. Its main uses are found in photography, litho work, and in offset.
Oxalic acid is also found in crystal form, is water soluble, and is very caustic. It is used to prepare metal polish and is also a component in steel mordants. Some last additions to this list of acids are: tartaric acid, used in photography and litho-offset, qallic acid, and hydrofluosilic acid.

A. SODA. Caustic soda is sold in stoppered flasks, as a solid or in concentrated solutions which are called lyes. Two different types of soda are available, the first being washing soda (also known as Solvay soda) which is the raw material, while alcohol soda is the purified version. In its solid form soda absorbs humidity and therefore must be kept in an air-tight plastic box and not in bottles with a ground-glass stopper. The lyes are easier to use and to keep (as well as being cheaper) and can be had in a variety of concentrations: 36°B which is equal to 33% pure soda (washer's lye), 40°B (37% pure), 45°B 42% pure), and 50°B (50% pure). These lyes avoid having to make solutions with pure soda which is not only a delicate task but also dangerous. In any case one should avoid lumps of soda that have to be crushed and might cause slivers to be projected into one's eyes. For much the same reason one must not prepare a lye solution with hot water as this might cause overheating and spattering. If one is to prepare a lye solution this must be done by adding small amounts of soda to cold water and mixing everything vigorously. Measurements should be made with a Baumé hydrometer which is always at the same temperature. In case of burns, rinse the burned area well and then apply a plaster of borax or sodium bicarbonate. Spots on one's clothes can be neutralized if they are cleaned immediately with a slightly acidulous liquid.
Caustic soda will attack aluminium, zinc, tin-plated and galvanized iron, enamel, and all organic matter. It can be used for bleaching, to clean, to unclog workshop pipes, and, above all, to scour metal plates (in this case it should be mixed with some whitening), and to wipe* the sides and margins of plates or the ink left on the whites of plates before printing. The solution used to remove painted or grounded plates is, at times, sold under the misleading name of "potassium". A liquid solution of soda is also used in silkscreening to clean screens (screens should be cleaned "out of contact") and to remove residual grease before applying films. Finally, it is also used in photographic developing processes.

B. POTASH. Caustic potash can be had in much the same form as soda. In its solid form it is even more sensitive to atmospheric humidity than soda. Two main potash lyes are available: one at 36°B (44,5% pure) and another at 40°B (52% pure). It has almost the same characteristics as soda but it is somewhat more expensive and thus less often used. It may he pointed out that 56 grams of potash will replace 40 grams of soda.

C. AMMONIA . Ammonia is a concentrated solution of ammonia gas and has a strong distinctive smell. It may be bought in the following three concentrations: 22°B (20% gas, which is the alkali), 25°B (27% gas), and 28°B (34% gas). It is a very volatile substance that must be kept in a flask with a ground-glass stopper and in a cool place. It is used as a cleaning agent in litho-offset.

3 . SALTS.
Salts are formed by acids when they are combined with bases. Each acid can be used to prepare various salts. As a consequence there are many different salts but not all of them are used in print making. As in the case of acids, they can be sensitive to humidity or be efflorescent.

Nitrate deposits were used in Spain and in the Middle East to attack metals and decorate weapons much before the use of nitric acid itself. Potassium nitrate, also know as saltpeter, aluminium nitrate, and sodium nitrate are used in litho and offset work. They are all water soluble powders. Cellulose nitrate is used in making the light sensitive colloid (gel) layers used in photographic transfers* onto metal. Lead nitrate is used to reinforce blocks (plates), while silver nitrate is used in making plates light sensitive.

This category of salts is not much used in print making. Copper sulfate was used in the past to colour the water in large bottles used as reflecting screens to light up the plate being worked on (see verdigris below). Aluminium sulfate is a whitish mass which is soluble in water and used in gluing paper and in preparing lacquers. Sodium sulfite is used in bleaching paper and in photography. Sodium bisulfite is used in litho and offset work while sodium hyposulfite is used in photography.

Calcium chloride is used as a drying agent and is placed in graining boxes ( in aquatint work*) to keep the resin as dry and powdery as possible. It is also used to keep photo-engraving and offset plates dry when they are put in hermetically closed boxes. Calcium chloride can be had in solid or grain form and can absorb a considerable amount of humidity. Antimony trichloride is used to put a patina on ferrous metals. Mercury bichloride is a colourless crystal soluble in water, alcohol, and glycerine which is very poisonous. It is used in photography, to put a patina on metals, as well as to bite aluminium* plates. Zinc chloride is used in scouring.
In the past sodium chloride (normal table salt) was used as a mordant component (along with rectified vinegar), and as a grain in preparing the granulated* surface in aguatint*. Ammonia chloride, also known as sal ammoniac, is a component in the etching mordant described in Abraham Bosse's treatise on engraving along with vinegar, table salt and verdigris. It is also used in steelfacing along with iron sulfate. Ammonia chloride is also used with verdigris and salt in making a honey mordant* which is used as fine biting mordant for copper plates.
The last one of this category of salts which will be discussed is
iron perchloride which is one of the most used salts in the graphic arts (especially in aquatint ), in photogravure, and in putting a patina on metal. It merits special attention here because of all its uses.
Iron perchioride is to be had in the form of yellow or brown crystals which are extremely sensitive to humidity and must, therefore, be protected from any contact with air. In the past photogravure makers made it themselves in order to be sure of getting good results. The chemist Bonnet, author of a treatise on photogravure, gave the following recipe: "Put some nails and tacks in a large earthenware pot and pour onto the iron nails and tacks a orixture of hydrochloric acid and nitric acid which is the same as the one used in "aqua reqia" (one volume of nitric acid to three volumes of hydrochloric acid). The work must be done in open air because after a few seconds the contact between acids and iron will produce a torrent of hyponitric acid fumes which are very harmful. The mixture should be stirred every once in a while. If necessary, the mixture can be heated to help the reaction along but this will not always be necessary. The result of this reaction will be a brownish-yellow liquid which is quite thick and whose content will be mostly iron perchloride with only some iron nitrate and a small quantity of free acid, etc. It is an excellent mixture for photogravure and even though less pure than what is bought it will be found to be superior". Today there are also coramercially available products that are of excellent quality.
In general, the necessary mordant solution is made using crystals even though it is possible to find ready made liquid solutions. The solution is made by mixing 10 kilos (22 lb) of crystals with 3 liters (3,3 quarts) of water at 15° Centigrade (122° Fahrenheit). The solid iron perchloride is put into a plastic, rubber, sandstone, etc. recipient (never use a metal one) with an opening big enough to permit mixing the ingredients (mixing should be kept up for some 20 minutes). After cooling off weigh a sample of the liquid obtained and lower its concentration (by adding water) to 46°B. The job has to be carried out carefully since the solution obtained by the above mentioned mixture will be anywhere betwenn 50° and 48°B. Be careful in adding water because the concentration decreases very quickly. Remember to shake the mixture and make sure that it is between 18° and 20° Centigrade (64 and 68° Fahrenheit) before using the hydrometer. The solution is right when the liquid reaches the 46° mark of the hydrometer. Measuring should be done in a tall container whose thickness is proportionate to the quantity to be measured. The pasty deposit produced by this first mixing should be kept to be used in reinforcing solutions or in making new ones. Any handling of this salt should be done carefully and only when wearing rubber gloves. In fact, iron perchioride is quite caustic even though it is not as corrosive as strong acids. It will turn hands yellow and spot and attack clothes. A 46° solution of iron perchloride should be prepared in relatively large quantities since it is used for most mordant baths (as in aquatint) and in making 41°, 36°, and 33° solutions which in turn are used for complementary bitings, intaglio etching, scouring, etc. The bottles containing the various concentrations should be properly labeled. Furthermore, the solutions should be "aged" before use either by adding and dissolving 2 grams of copper shavings per 100cm3 of solution or by mixing the new solution with 5 or 10% of an old one. The reason for this is that too "fresh" a solution will bite too much and too superficially. The mordant is at its best when it has turned brown which is an intermediate stage between being translucent and being quite black. In the course of its use the mordant will eventually be exhausted. This happens as it dissolves metal and augments in weight. In the case of copper it will increase by 1°B every 10 grams of copper dissolved per liter, but, at the same time, the concentration of pure iron perchloride will be lowered by 10%. A mordant that is worn out will bite slowly and irregularly, which is the sign that it should be replaced. Part of the old mordant bath can be kept to ensure constant concentrations (10 to 15 grams of copper per liter are the right amount). The age of a mordant and its copper content can only be told by the colour of the solution. A "fresh" mordant is reddish brown and translucent and becomes brown and murky after a few bitings, slowly turning black with use. At the very end it starts to produce greenish veils. These various stages correspond to the following copper centents: 15 to 20 grams of copper per liter when brown, up to 50 grams per liter when black, and 35 or more grams per liter when greenish ( which is also when the solution becomes unfit for use). If needed, one can keep samples of mordant (whose characteristics are known) in test tubes and use such samples to compare them with the working solution. It will be found that it is best to let a freshly prepared solution age a couple of days before use.
The solution should be kept in closed bottles or pans so as to avoid contact with air. Furthermore the sotutions should be stored at the same temperature they are used at. In modern times ferric chloride (=iron perchloride) is more popular than ever because of its "less toxic properties. Especially when used in vertical etching tanks some problems of sedimentation of cristals have been overcome. The
Edinburg Etch is based on the use of ferric chloride with some citric acid added to it. It enhances the quality of etching and speeds up the process. Further details as to the use of this solution will be found under the headings aquatint ,etching , mordant,and biting .

D. PHOSPHORIC ACID SALTS. Sodium, potassium, and ammonia phosphate are sometimes used in litho and offset work.

E. HYDROFLUORIC ACID SALTS. Sodium fluoride is used for glass etching while ammonia fluoride is used in Iitho-offset work.

F. CARBONIC ACID SALTS. Lime carbonate, commonly known as chalk, is commercially availahle under various names such as Meudon white. It is used as a pigment and in scouring preparations, in which case it is wetted with lye. Soda carbonate, sometimes known as Solvay's soda, is used in photographic work and as a neutralizer against acid burns.
Finally various other salts and other compounds must be mentioned.
Neutral copper acetate used to be made by oxydizing copper with vinegar and is nothing else than the verdigris which Bosse called " verd de gris ou verd de cuivre" and used as a component of his hard ground for etching. Bosse also added that verdigris should not have bits of copper and grape pips in it. In fact verdigris was made with fermented grapes in the region of Montpellier. Verdigris when bought, comes in the form of blue crystals that are highly soluble in water and will turn the solution blue or greenish blue depending on the concentration. Such solutions were also used in large bottles to diffuse light in much the same way as copper sulfate (see above). Potassium bichromate, on the other hand, is used in litho work and as a component of mordants used on soft etching grounds*.

general comments

As we have seen, the various products examined above can be used in a variety of ways. It is for this reason that they have been grouped under headings rather than examined separately.
These products can usually be had in various forms, either solid or liquid. In their solid form they might be deliquescent, i.e. they naturally absorb humidity from the enviromnent. On the other hand, they may also be efflorescent if, conversely, they tend to loose their humidity.
Some of the abovementioned products are dangerous so that storing (to be reduced to a minimum) and use must be accompanied by precautions which can sometimes cause responsibility conflicts with insurance companies. Some acids can spontaneously burst into flames when in contact with paper, straw, etc.
When speaking of the concentration of a solution the measurements are always given in terms of Baumé degrees and are measured with a hydrometer graduated for such measurements. The liquid should be put in a graduated test-tube and the reading made at the level reached by the liquid on the floating hydrometer. For highly diluted acids one must use a hydrometer that will read down to O° Baumé. Remember that measurements must be made at the same temperature since increased heat will alter the readings [* measuring].
When mixed with water, some strong acids will release a lot of heat and this phenomenon must be kept in mind when measuring their strength. ln such cases the solution must he allowed to cool down before proceeding to measure it.
Acid must be poured into water and not the inverse in order to avoid dangerous spattering. Pouring should be done slowly as an abrupt increase in heat (as with sulphuric acid in particular) may crack the container being used. Solutions should be kept in containers which can he property closed and properly labeled so as to avoid confusion and delays.

Back to Main Page of Technical Dictionary of Printmaking