Characteristics of acid etching

Associate videoes do not exist

The acid is perhaps the main protagonist of the technique of etching. During its long coexistence of more than five hundred years with the recorders, has been respected, loved, needed and now, for reasons of sustainability, is repudiated by many giving way to less toxic substances and other pollutants. A nothing to read a little about them will confuse the variety of formulations, but in reality all the same function: to dig into the metal drawing by the artist on the layer of varnish. Take a look back at the entrance of the etching.

I take time thinking about how to address this post because of its size, the number of items that are in print and on-line, by the different ways to proceed according to the solutions and the amount of metals used that vary according to tradition, geographic location, or simply the preferences of the recorders. It is expected therefore that this issue again increasingly addressing recurring basis from a different point of view. Today I will focus on some generalities of acids based on their performance and features from the point of view of the engraving, with the intention of increasing the knowledge applied in the workshop. Not impact too much on formulas or preparations as there are many pages written, although it will indicate those sources seem more appropriate to expand the data here to see you today.

Acids are corrosive substances must be handled with care. You must protect your splash, especially in the eyes and wounds, avoiding any contact with the skin. Burns produce you can corrosion, so you have to handle taking the proper precautions and always under the strict rules of common sense and responsibility. Follow the instructions to give you the expertise you work in a shop and have as the highest, if not know anything about their operation, it is always best to ask first. On the other hand, the necessary respect should never become excessive fear drive you to act with unnecessary sudden movements that may cause undesired situations. I always use the example of a girl who abruptly dropped the iron on the tray, to fear that he was playing acid on the hands, and fell flat on the metal liquid splashed flamboyantly.

By definition, an acid is a substance that reacts with metals dissolving and releasing hydrogen gas (hence toxicity) so make it fit in very well ventilated. Carbonates also acts as marble, terrazzo ... so you should also put protecting natural surfaces such as floors, enamel bathtubs, and so on. Neutralizes bases and presentation will be solid or liquid. In the aqueous solution has a pH less than 7 always.

A basic rule to indicate when it is a solution of this nature is that you always check the water first and then to avoid the cumbersome acid reaction occurs due to intense heat (boiling delayed) causing serious splash . Given the importance of this clarification and to avoid forgetting there even mnemonics as indicated in the Faculties of Chemistry "him about it ... do not be evil-minded. I will discuss basic to a laboratory on the use of acids when it is diluted. There is a rule that I explained in the first year of study. The girls and us macho seemed practical. As our professor of biochemistry, "remember, always him about it" (the acid into the water). Great advice .... Other less widespread alo serve the same "First the water, then the acid, otherwise it will not be placid" Although this exothermic reaction does not occur with all products, follow it as the norm and you will avoid unpleasant surprises.

Also an acid is "the substance added to water does increase the concentration of H3O +" (this is the formula for the hydronium ion is formed when you mix an acid in water: the H3 symbolizes that has three hydrogen atom, O to has an oxygen atom and the + (superscript) that has a positive charge, or who has lost an electron). Water is who activated their reaction function as a powerful catalyst. In this respect, among the characteristics of acids that are important to a recorder should be known that there are oxidizing acids or oxyacids, such as nitric or chloric, and oxidants (such as hydrochloric or phosphoric acids, for example). Oxidizing acids are more toxic in a non-oxidizing reaction by the gases released. In addition, in some cases, the fatty oxyacids corrode the metal, and sometimes this will affect the clarity of the bite to the oxide deposited on the plate itself that becomes passive when attacked by the etching away with imperfections.


There is also strong and weak acids and not all act the same way with metals. This has led to describe them by saying they have the property of producing "a vertical bite," meaning if the acid attacks only the depth of the size or even their walls. This responds to the same virulence, if excessive, will affect the sides quickly opening the incision and peeling varnish. After a while the lines are together producing the so-called "bald" bite very broad areas that fail to retain the ink to be drawn by the tarlatan.

The strength of an acid, as not all are equal, depends among other things, the dissociation constant "Ka" means that the acid dissociates into two ions, one positive and one negative, when in contact with water. And depending on this constant makes it more, less or completely divided into strong, intermediate, weak, very weak. For example, in a vat of nitric acid concentration equal to that of a bucket of hydrochloric acid (main ingredient of Dutch mordant) to get two plates of the same size we know that the HNO3 dissociates to a greater extent than the HCl attack and therefore more strongly to the plate. It follows that the bite still further dissociation acid makes it more active and therefore will be spent before the HCl which has a lower degree of dissociation. In short: This is one of the reasons that stronger acids are spent before the weakest.


When we call recorders strong or weak acid we also refer to the concentration will vary depending on the techniques and the speed with which you want to bite: strong acids or concentrated solutions are used for terracing techniques, hypermarkets bites, thick lines ... and weaker concentrations as tightly woven delicate work of lines, aquatints, and so on. To accurately measure the concentration or density of a solution is to use a hydrometer to give a value in degrees Baume. But realistically the weight acids is not an artifact that students tend to have recorded, not many writers in his workshop, making it according to the formula provided in the different media. In this regard I must say that although the solution and ensure that, for example, with respect to ferric chloride (salt widely used a mordant), there dissimilar versions of how you get a saturated solution (38 º - 40 º ). Some talk of a 40% solution while others propose five volumes of an acid in water and density that marks the pesasales not fail. With regard to nitric or Dutch is easier: strong solution of nitric call a completed around 50% and about 15-20% weaker. In hydrochloric acid are added volumes (see the formula that will put more below). From this we must deduce something essential: the variables that affect the bite of an iron are many, to this is that small variations in time (after the first few minutes) is marked not large differences in gray, so the formulas vary from other recorders that fit their particular ways of drawing ... but now we return to this issue.

There are certain factors that affect the speed and strength with which an acid acts on the plate. These causes result in sentences that are heard in the workshop such as "little bites today," "as more acid bites," "must be activated" "hot, watch out" or "what's wrong with that today" we lost in jargon difficult to pinpoint reactions and sometimes given to engraving aura of mystery and difficulty. Without wishing to disappoint the romantic mystery is not so much as chemistry and all, or almost everything is explained. Just causes the recorder to the variety of casuistry returns to his genuine concern for most artistic matters.

The first and foremost factor that influences the reaction rate is its concentration, ie, the amount of pure acid found in the solution. Thus a 50% solution of HNO3 have half of nitric acid and half water, a solution of 10 percent of it will be a part of acid HNO3 and H20 and 9 can be expressed by weight or by volume. How does this affect the bite of the plate very well explained to his fellow Jose Manuel, a former student who graduated from the Faculty of Chemistry and came to be called the "theory of the balls." He said: "Imagine that every molecule of acid is a ball that moves through the tank and when it hit the metal reacts and bites. In this way is not the same as in the bucket has 100 balls (say a number) that would be the case at 10 percent, which has 500 balls, which is the case of 50 percent. In this solution the collision with the metal is more crowded and therefore the reaction rate increases.

"the same reason the concentration of acid is becoming a less and 80% in contact with the metal will react very fast ... very fast but also lower concentration. So the lower the longer it takes it down, so the weaker acids solutions and last longer. So, following the example earlier of the two acids, in the same period of time while the acid has dropped 50% to 20%, 10% has dropped to 8%.


The next factor is the temperature, at which following the example of the balls ... "if we imagine that the balls of acid in a bucket are moving and that this movement collide and react with the metal, we can think more slowly moving slower to hit the iron. With increasing temperature also increases the kinetics of molecules producing more crashes in less time. It happens, therefore, as with the concentration: the higher the temperature shocks, so the low acid concentration and wears faster before. "

Another feature is the specificity for different acids as not all metals are attacked with the same intensity. Contrary to what one might think, when he gets a plate of a given metal in a bucket of acid more compatible does not mean it the better, as may agree not to use when needing a softer bite. You can find on the net several tables of this nature. I just now named the specific solvent for copper, silver and lead is nitric acid, the aluminum or tin and hydrochloric acid zinc or iron all the acid in general.

Another factor that concerns us as writers is that of the catalysts, what are they?. A catalyst is a substance that, even in very small quantities, varies greatly speed a chemical reaction without noticeable hardly any change in itself. These are substances that are neither original nor reactive end products. An example of catalyst is potassium chlorate in the Dutch mordant, a salt that is poured in small quantities and produces a large increase in the rate of reaction of acid (in this case the catalyst is potassium). Sometimes, the catalyst may be the metal itself bathing in acid oxide per share and therefore it sometimes will take a metal piece in the bucket even "activate". To the contrary, there are substances that work the opposite way to the catalysts, and these are the inhibitors. It may happen that you accidentally dropped an inhibitory substance in the acid and this slow down the reaction, although not very frequent. These substances neutralizing agree as a last resort when disposing solutions. [There are other factors that influence the reaction rate as the pressure or the physical state of the reactants but no interest to us]
The number of acids that can be formulated is quite broad but not all work well for engraving, nor are all user friendly, nor is it easy to obtain or just recommended for health, so that the broad arc is closing to go to stop the most commonly used. The best known and used are nitric acid (the most toxic and virtually obsolete ... or at least should be), the Dutch mordant (very irritating) and ferric chloride, which is really not an acid but a salt , considered the least toxic of the three. I will give a basic description of its use but if you want to expand your knowledge go to the text engraved sustainability written by three top experts on the subject: Eva Figueras, and Cedric Green Kiekeben Friedhard for the book non-toxic printmaking, new procedures and materials where you can find valuable information about their history, usage, composition, preparation methods, safety standards, tips for recycling and so on. I find it essential reading for any writer.

NITRIC ACID: In etching is often used for copper and zinc in solutions called strong (around 50%) and weak 15-20% This substance is colorless fuming when new and generates a series of bubbles of hydrogen and nitrogen, very toxic, you have to be shed from the surface of the plate so you will not spotting as moles. The ancient writers used a quill. Given the toxicity, even though more traditional acid, it is becoming less used. Very virulent reaction to any change of temperature, level of work, etcetera. Not recommended. The teachers always used to drink milk or juice advise when it was a long time in contact with him.

MORDANTS DUTCH: a solution based on hydrochloric acid, potassium chlorate and water, the formulation varies subtly from one author to another. The Complete Guide proposed engraving and printing techniques and materials of John Dawson, H2O is nine parts, one part of hydrochloric acid and potassium chlorate fifth "crystals are mixed potassium chlorate with some water until all crystals are dissolved, then bursts into a bucket the rest of the water and add hydrochloric acid, and finally poured into the bowl the mixture of chlorate and water. Doing so will emerge which fumes inhalation should be avoided. Let stand a few minutes. "


Hydrochloric acid is obtained in the laboratory by adding sulfuric acid to common salt. The vapors that come when we mix the two solutions are hydrogen and chlorine gas, which is as a catalyst is potassium as reagent and the rest of hydrochloric acid. Of the hydrogen and chlorine fumes irritating understand what the mix. Although it is highly corrosive gastric juices used 3% of this acid to function and is the basic component of domestic salfumant, etching, formerly used for unblocking pipe ... luckily disappearing custom.
The Dutch mordant in its composition has a fundamental solution of hydrochloric acid and chlorate of potash in water, in proportions: weak solution: 88 vols. of water, 10 of hydrochloric acid and chlorate of potash 2. Strong solution: 88 volumes of water, 20 of hydrochloric acid and chlorate of potash 2. Its advantage is its transparency, which disappears as it is used, turning a bluish color, but is highly irritating to eyes and respiratory system, so if you do not have a well-ventilated place for use, opt for another solution . I'm telling you from experience. Also if you store it in a workshop with its simple evaporation can oxidize metal tools and enclosures. It evaporates in the heat.

There are also those acids combined as "aqua regia" used once, which is a mixture of nitric and hydrochloric acid on a one to three (1HNO3 + 3HCl)

FERRIC CHLORIDE: for bitten plates are also used corrosive salts such as ferric chloride (FeCl3). Use of this product is recovered by being the least toxic of all because it really is a Lewis acid, ie reactive activity is not the exchange of hydrogen ions (which is typical in acids) but for the exchange of electrons getting their acid load of metal and hydrogen. Thus, this transforms the iron mordant salt chlorine, so it disappears and appear metallic copper waste, its main drawback. See that this is a very opaque acid produces a slime that can clog even sizes, why bite the plates upside down. Also known by perchloride of iron, although the salt would be a more correct to call it iron perchlorate. Another synonym is correct iron trichloride.
To prevent the generation of this waste Kiekeben devised, among others, the Edinburgh mordant for copper is precisely this: 4 / 5 of ferric chloride saturated, 1 / 5 citric acid solution (consisting of 3 / 4 liter of water + 1 / 4 anhydrous citric acid (powder). for example, to a large amount: 6 liters of ferric chloride (40%), 1,200 cm3 of warm water in which they throw 400 gr. of citric acid powder then pour in the iron slowly. It is often stronger than ferric chloride without citric is cleaner and can chew up the plates, although this does not prevent a thorough rinse out each time the cell plate. Do not forget to also monitor occasionally a possible occlusion of the carvings.
In this respect there is a significant fact to be observed and pointed out already above. One of the data they have found more versions is the amount of ferric chloride required to make a saturated solution. We have found from 2500 grams in a liter of water, 400 gr., 500 gr. to the point Kiekeben appoints himself the variety and the need to make a particular adjustment in the formula. Theirs is to buy already diluted, but if they are used as the most reliable pesasales. You will find that there are always variations, all valid in principle, to work. We have worked with solutions of 400 and 500 grams in a liter with very good results with the solution already made​​, with the bite of Edinburgh and even adding lemon juice to the mixture. These variations among some formulas and some do not seem relevant to obtaining the results we seek recorders.

You should make a test strip that responds to a table of results as to how each artist drawing in particular. Usually tends to establish a wide range of time bite with a certain acid, metal frames and several more lines less closed. This test strip lets you know how long aproximadmente require your different grays, and if you look closely also possible to establish when and how to draw produces bald. You must take into account the type and the time it takes the acid prepared, drawing, room temperature, the different techniques ...

I wish you luck, I advise care, I encourage you to overcome difficulties and likely end up with one sentence: no one can save you the experience you need to be a writer

[Fuente imágenes: superior: propia, cubetas dentro de la campana de extracción de ácidos del aula de grabado de la Facultad de Bellas Artes de Sevilla;  imagen niño en ite.educacion.es; siguiente izquierda: representación del ión hidronio en Wikimedia; siguiente: esquema de plancha sumergida en ácido en; grabado antiguo de A. Bosse donde puedes ver grabadores trabajando en las cubetas de ácido en eureka.ya.com ; molécula de nítrico en windows.ucar.edu;  pesaácidos sumergido en cloruro férrico en patrisdelgado.blogspot; plancha de cobre mordida en gravat.cat; molécula de ácido nítrico en wikipedia; plancha en cloruro en patrisdelgadoblogspot(op. cit); tira de pruebas en mordiente de Edimburgo realizada por el alumno Manuel Naranjo, dirígete a su blog para ver todo el proceso es muy completo. También quiero agradecer mucho de lo hoy escrito  a las aclaraciones que me hicieron tanto a José Manuel, de quien no consigo recordar el apellido, como al gran amigo y químico Antonio Romero Lóriga, a quien he bombardeado en multitud de ocasiones con mis preguntas]

Maria del Mar Bernal (professor of engraving at the School of Fine Arts, University of Seville)


María del Mar Bernal revisa periódicamente este y otros artículos publicados en el blog de la Universidad de Sevilla.


Associate attachments do not exist

Return to the list