2. DIFFERENCES IN THE BITING PROCESS.
A. D1FFERENCES DUE TO THE MATERIALS USED. In general the choice of the materials to be used is dictated by economic reasons, by the ease with which the work is done, by the quality desired, and by the resistance of different materials to the printing process. For example, steel used to be preferred because, when properly treated, it could make as many as 50,000 impressions and, at a time when steelfacing was not yet known, such a quantity of good grade impressions was practically impossible to obtain by any other means. In fact, as is well known, even well worked copper plates begin to show signs of wear as of 1000 impressions and usually cannot be used for more than 2000 or 3000 impressions.
It is, however, possible that a print maker chooses a particular metal because that particular type of metal seems to him to be the one which is most successfully bitten or which gives the best results. These are the reasons for which copper enjoys such a good and well merited reputation for intaglio work (both etching and photogravure). The steelfacing and chromefacing processes carried out on copper plates further enhance the inherent qualities of copper. On the other hand, insofar as relief printing is concerned (a process in which plate wear is reduced to a minimum), it is often done on zinc plates which can be etched very quickly.
Thus it is up to the print maker to foresee, amongst the reasons determining his choice, those which favour one metal rather than another in function of the etching process he will use.
It might also be pointed out that despite the abovementioned considerations various attempts have been made to use different metals on the same plate. In a process invented by Vial in 1863 the drawing is done on a zinc plate using an ink which is saturated with copper sulphate. When this type of plate is etched (using a weak solution of nitric acid) only the zinc is attacked and, as a consequence, the drawing will be higher than the rest of the plate. Other such systems exist using various metals and thus artists may use mercury base inks on copper plates or gold base inks to draw on silver plates so as to end up with either intaglio or relief plates.

B. DIFFERENCES DUE TO THE MORDANTS USED. We have already seen in various articles (attacking agents, mordants and articles devoted to various techniques such as aquatint, etching, photogravure, etc.) the importance of choosing the right mordant in terms of both its compositionand concentration. In fact, the range of attacking agents is such that the printmaker can choose from a large variety of etching possibilities especially when the material to be bitten is a metal. He can choose to bite rapidly or slowly, superficially or deeply, etc. It might be said that there is a chemistry of mordants that necessitates a certain knowledge and a certain amount of personal experience since the biting must, ultimately, meet the artist's personal needs. It is not a good idea to lose too much time in experimenting with very complicated formulas but, on the other hand, the artist will do well to search for the mordant best suited for the results he requires.
The search for a specific mordant has resulted in a variety of mordants of which the most important ones are discussed under the heading mordants. In order to point out the type of mordant used French etchers of the 19th century called their print a "fluorographie" if it was done with hydrofluosilic acid or a "chlorohydrographie" if it was done with hydrochloric acid. In order to further complicate things some etchers even thought up methods of etching plates several times, using a different acid each time. The more delicate parts were bitten first using a concentrated solution of tartaric acid. The second step was then to stop out the areas bitten with tartaric acid and etch the plate with acetic acid, followed up with hydrochloric acid, nitric acid, and, finally, finished off with aqua reqia (nitric acid + hydrochloric acid) to etch the black areas. This type of etching process resulted in prints called "isographie".
The differences in mordant concentration allow the artist to bite the plate to the depth he requires. In this context let us recall the general principle whereby:

high concentrations = rapid but superficial biting
low concentrations = slow but deep biting

Iron perchloride, however, does not follow this rule due to its being a very viscous mordant when very concentrated. Due to its consistency highly concentrated iron perchtoride cannot etch rapidly.
It is by juggling different concentrations (and different layers of etching) that an artist obtains the various superficial and deep etchings he requires [* bite until boiling begins , etching, photoengraving].
The choice of a mordant may also he deterinined by more practical reasons. For example, some people cannot bear the toxic fumes produced by nitric acid and thus prefer to use iron perchioride, which is a good substitute.

C. DIFFERENCES DUE TO THE STOPPED OUT AREAS. Stopped out areas are the parts of the plate protected by a ground, a stopping out varnish, or any more or less acid resistant substance. It may be said that there are two different types of stopped out areas. The first one of these may be considered a discontinous stopping out in that it does not cover the plate surface uniformly but rather only the areas to be protected from the mordant. The second type of stopping out may be called a continous one in that it covers the plate entirely even though it is permeable to the mordant in different areas. An example of this second type of stopping out is a gelatine layer which when exposed to light becomes permeable or impermeable in function of the positive and negative areas [* photographic processes]. The choice of grounds to be used should be made in function of the etching to be done as well as in function of the lines laid before or after biting. There are many grounds available and each one of them serves a specific purpose: a solid ground laid with a dabber will be found to be much stronger and will resist longer than a liquid ground. On the other hand, a liquid ground is indispensable in stopping out the lines once they have reached completion. Acrylic paints protect the plate well but can only be used with a gentle mordant [ * aquatint]. The inks used in stopping out are particularly fluid and thus can be made to flow along the slope* of the lines that have already been etched [*ink, photoengraving]. Various kinds of gums and resins can also be used to ground the plate and, furthermore, reinforce the ink and varnishes used [ *lithography].
The danger of discontinous grounds is that, due to the fact that the biting process tends to spread superficially and laterally, the edges of the ground may be damaged and a crevé or burst-through* may be provoked. The crevé or burst-through is said to have happened when separate lines, divided by a thin line of ground, merge under the influence of a mordant. Another problem to watch out for is the deterioration of the slope* and the ground's border. The solution to such a risk is a series of slow and separate bitings according to the lines to be etched (see sections A, D, E and F of this article for further details).
If an artist so wishes he can obtain differences in the discontinous grounds either by stopping out areas already bitten or by using grounds which are permeable to different degrees. For example, acrylic paint can be penetrated to a certain extent by a strong mordant hut when covered with a ground this same paint becomes entirely acid resistant. One can thus juggle with the relative porousness of various combinations of acid resists. The felt* tip process is another example of relative porosity since the inks used in felt tip pens will prevent the mordant from penetrating according to the number of layers applied. Most inks and lithographic crayons can be used to the same effect.
Continously stopped out grounds are different in that two separate phenomenae are simultaneously at work especially in the case of gelatine layers. Humidity makes gelatine swell in proportion with its exposure to water and therefore the acidity of a mordant will act according to its water content. The singularity of photogravure etching is due to the complicated juggling with a mordant whose water content is increased or diminished while at the some time the acidity of the mordant is being manipulated [ *aquatint,photogravure , photographic processes].

D. DIFFERENCES DUE TO MECHANICAL ASPECTS. Etching a discontinous ground can only result in a plate for a print. There are, however, two basic principles which must be distinguished. In the first place the etching can be done on a plate which has been made selectively resistant to the mordant by means of a greasy pencil or a brush stroke of paint, ink, varnish, etc. The second type is that in which the plate is first covered with a ground and then drawn or worked upon. In the first case only protected areas will resist the attack of the mordant (to a greater or lesser degree, it is true). In the second case the plate is protected except for in the places where it has been worked either by pushing through the ground or then going so far as to work the plate itself (after going through the ground).
A variety of lines can be achieved, even though only one tool is used, if the plate is exposed to several bitings. On the other hand, if various tools are used (from the finest point which will only scratch the ground, to the biggest graver which cuts directly into the plate) it is possible to come up with a great variety of different lines even if only one biting is carried out. The latter method is a mixture of drypoint and engraving used in etching.

E . DIFFERENCES DUE TO THE BITING METHODS. The different methods adopted in biting a plate are chosen according to the greater or lesser ease with which a particular metal is bitten, according to the space available in the workroom, according to reasons of hygiene, etc. They are also chosen in function of the different results required.
Insofar as space is concerned it may be pointed out that biting by immersion generally requires more space and equipment than biting done by "damming" the plate or by the "badigeon" method. In fact the immersion type of biting requires a pan or basin that is larger than the plate to be bitten as well as a greater quantity of mordant than what is needed in the wax "damming" system. In the case of the "badigeon" method, which is also the simplest of them all, the only equipment needed is a stick with some cotton on the end of it. It must he said, however, that the results of this variant of the "feathering" system are not very even and thus it generally tends to be used only for supplementary biting. In the 19th century this system was used to bite grained photogravure* plates of small dimensions.
Insofar as the damming process is concerned it does not permit the engraver to shake the plate back and forth to remove the bubbles formed by the biting (which is essential when the ground used is fragile). Furthermore this process can only be used to bite small or medium sized plates.
An ancient method, which may be called the pourinq method, consisted in placing the plate on a kind of stand and then pouring the mordant from a jug directly onto the plate. This method had certain advantages as well as a few drawbacks. In fact a great deal of fumes are released by this method, which is a drawback if they arc toxic. On the other hand a long biting time is needed with such a method and this can present certain advantages. Insofar as the results of the biting are concerned one must distinguish between planographic biting (which requires several biting and may be done with several mordants but in which the ground is not touched) and bitings which may be called "covered" or rebiting ones (characterized by stopping out procedures).
Rebiting permits a lot of nuances but it also requires a lot of manipulating. In any case this kind of biting is almost indispensable and, in methods such as gillotype*, rebiting is used systematically.
When it is being bitten a metal can become covered with bubbles after a short time, especially when using certain mordants (such as nitric acid). These bubbles impede a further biting and thus they must be removed if one wishes the biting to be continous and regular. This job can be done with a pigeon father, a brush, etc. But it is not always recommended, especially when the ground is fragile. Better results can be obtained by rocking the mordant bath back and forth. This can be done by putting a crayon under the biting pan and use it as a kind of pivoting point. The back and forth rocking motion cannot, however, be used when the biting is done in the "damming" manner. In photo-engraving work there are mechanically oscillating vats which obtain results identical to the more artisanal method described above.
The reader's attention should be drawn to the different positions that can be given to the plate when it is bitten. The horizontal position most often adopted is not the best one. Actually the best position for biting is that in which the plate is horizontal but the image side is facing down. However, such a position is not very practical. An intermediate position in which the plate is at an angle is not too difficult to set up and presents certain advantages. In fact, the pouring method mentioned above used such a position. On account of the slant the bitten metal particles were washed away while at the same time the used mordant was removed.
There is another biting method which is sometimes described in specialized literature. This method consists in biting a plate immediately after having wetted it with water. I have only reminded the reader that such a method exists but will not describe it in greater detail as it does not seem to me to permit a controlled biting.
Finally, there is the galvanic* (electrolytic) biting method which is extremely regular. This method is very delicate and requires a certain amount of equipment [galvanic processes (2, B)]

F. DIFFERENCES DUE TO BITING CONDITIONS. The conditions which affect biting are the time allowed for biting, the temperature at which it is done, and the atmospheric humidity factor. The last two conditions or factors are important only in the sense that they help biting but do not really alter the results. The right temperature for biting is around 20° Centigrade (68° Fahrenheit), 22° Centigrade is acceptable ( 71-72° Fahrenheit). The temperature of the mordant should never go below 18° Centigrade (64° Fahrenheit). Insofar as the atmospheric humidity is concerned it should be around 60% for best results.
The length of time for biting is all important and it even can be said that the quality of the results depend on this variable. But even though the time is a determining factor, and as paradoxical as it may seem, caution requires that only approximate figures be given for the length of time a plate should be bitten. In fact the amount of time a plate is bitten not only depends on the quality of the mordant (strength, freshness, heat, etc.) but also on the material being bitten, its position, the movement of the mordant bath, the size of the pan in which the operation is carried out, the weather (thunderstorms make mordants act capriciously ) , etc.
Biting time can range from just a few seconds to several hours depending on the concentration of the mordant used. An artist must also take into account the resistance of the ground being used and the size of the lines being etched. The reader will find some indications as to the amount of time usually allowed for in various types of etching in the articles devoted to aquatint, etching and mordant. It must be remembered, however, that the times suggested are to be taken as approximations and that only a certain amount of experience in the matter will give the necessary working knowledge. Having pointed out the limitations of any estimate of biting time I can now proceed to say that, when using the more commonly known mordants, a five to ten minute biting (in one bath) generally results in fairly dark printing lines. The general rule is that the more vigourous a mordant is the more limited the biting time must be especially if the lines are to be thin.
Of course time plays a yet greater role if the biting is done by immersion and expecially if a series of different mordants are used. The following example of biting time is calculated in terms of a 20° Baumé solution of nitric acid:

• 1st biting: the whole plate 1 minute
• 2nd biting: after stopping out the finest lines 4 minutes
• 3rd biting after stopping out the finer lines 4 minutes
• 4th biting: after stopping out the middling lines 4 minutes
• 5th biting: after stopping out the thicker lines 4 minutes

3. THE BITING PROCESS.
A. BEFORE BITING. The biting of a plate is certainly one of the most delicate operations in print making. Although it may not be physically dangerous for the artist himself it certainly is dangerous for the plate. A biting that has gone wrong is often the sign of an etching that has failed but it must be added that failure may be due to both excessive biting as well as to insufficient biting. In fact if too much biting has been done it will he quite difficult to correct such a mistake. When the biting is insufficient the artist may not notice it until he has removed the ground from the whole plate (a corner is often enough). In such a case the result is a light or "blond" biting, which means that the plate will be only slightly etched and even if it is rebitten it will result in a stilted image as it will have lost the spontaneousness usually characterizing the best prints. In most cases it will be found that it is best to abandon an under-bitten plate and start again with a clean plate. In other words, the artist can hardly be careful enough when biting his plates.
In order for etching to be carried cut in the best conditions the artist must have enough space to work in so as his movements not be cramped. First of all it is essential to have enough light in order to be able to check-up on the biting process while it is under way. The etching pan should always be bigger than the plate so that the particles ef metal already bitten can be properly dispersed in the mordant. The sides of the pan should he high enough so that a rocking motion can be used during biting if one so wishes. To facilitate the rocking motion place a rod or pencil under the middle of the pan. The mordant can then be poured onto the plate which is already resting in the pan. The mordant should just reach the level of the plate as it rests in the pan, so that the back and forth rocking movement will create a wave of mordant which will both bite the plate and then remove the used mordant from the previous wave. If the artist prefers he may brush the bubbles away using a feather or a hrush with long synthetic bristles. It this second method is adopted (which is only possible with a resistant ground) the amount of mordant may be increased so that it will be exhausted more slowly. In any case an immobile biting technique is quite unadvisable with dark coloured mordants, especially when using iron perchioride, since it is impossible to watch the biting white it progresses.
The biting pan should be as close as possible to a sink with a drain and running water. The reason for this is that the biting must be stopped at the right moment. In order to stop the biting immediately running water is preferable to a water basin no matter how big the basin may be. In fact a jet of water (diffused but quite strong) should be directed onto the plate for quite some time as this is the only satisfactory way of washing the mordant off the plate. The workroom or shop should be well heated but not dry.
Mordants should be prepared before they are used [* attacking agents and mordants]. The concentration of each solution should be clearly written on each container so as to avoid making mistakes and in order to work more rapidly. Another requirement is that the working area be properly aired, especially if toxic mordants are used. While on the subject it may be said that the toxic vapours are not always uncomfortable to breath in. On the other hand one should not forget that a bad reaction to high concentrations of a mordant can always be avoided by switching to more dilute solutions or even to non-toxic mordants such as iron perchloride. In order to ensure a good biting of a metal plate it must be properly scoured* (see article concerning this operation). It is quite easy to prove to oneself the inefficacity of biting without scouring. Just rub part of a clean plate with a dirty finger, clean the spot with saliva and then bite the whole plate to see the results.

B. DURING BITING. In print making there are four distinct techniques which use a biting process but here we will limit ouselves to the technique of etching. For details concerning the other three techniques the reader may refer to photo-engraving, lithoqraphy(for etching done with an acidulous solution on stone) and photogravure . The reader may also consult aquatint , crayon manner, and etching, for further details.
Before doing any biting it is essential that the back and sides of the plate have been properly protected with a stopping out varnish [ * stopping out the borders] Whatever method of biting is used (pouring, immersion, etc.) the goal to aim for is a maximum regularity in biting (unless, of course, certain parts of the plate are to be bitten individually). In any case, if a regular biting is needed the mordant must be quite homogenous so that all the areas of the plate will be bitten equally. Biting done with the "badigeon" method is the least satisfactory as far as regularity is concerned. As for the pouring method, one must remember to turn the plate around so as to compensate for the fact that the top part will always be less bitten than the lower one due to the fact that the mordant accumulates around the bottom.
Biting should always be begun decisevely and if a plate is put into the mordant it should be immersed immediately. If the mordant is to be poured onto the plate one must not hesitate. The pouring of a mordant should be carried out with the plate at a sharp angle so that the mordant flows well and does not form puddles.
After a period of time, which may be long or short in function of the strength of the mordant, the plate will blacken and be covered with bubbles. This is the signal to clean the plate and the lines being etched either by rocking the mordant bath or by brushing the plate itself.
It is quite difficult to judge how long biting should be prolonged. Some of the signs that show when biting is done are when the plate surface begins boiling or when the ground begins to lift (especially along the borders). With experience the etcher will learn for himself when biting may be considered to be done. In any case beginners should be very careful when carrying out their first bitings.
In order to assess the depth of the biting uncover a little bit of the ground, in a corner of the plate, and then measure the depth with your fingernail or then with a line tester.
Insofar as corrections and rebitings are concerned the reader may turn to the article on etching.