Etchings and the etching process explained.
Dutch Mordant on Copper plate
Part 2
The etching Press
The main hazards in photogravure etching and related techniques are strong acids and alkalis, organic solvents, dusts and powders, dichromates, and ultraviolet light. For some of these, non-toxic substitutes are available, and for those that remain, rubber gloves, lab aprons, and ventilation and/or inhalation masks are recommended.

Acids. Strong acids react vigorously with almost anything they touch, including skin, eyes, lungs, and internal organs. Dilute nitric acid and Dutch mordant (hydrochloric acid plus potassium chlorate) have traditionally been used in etching. About nitric acid, the Centre for Safety in the Arts (CSA) says: "Concentrated nitric acid is a strong oxidizing agent and can react explosively with other concentrated acids, solvents, etc. Nitric acid gives off various nitrogen oxide gases, including nitrogen dioxide which is a strong lung irritant [though odourless] and can cause emphysema. Large acute overexposures may cause pulmonary
edema (chemical pneumonia), and chronic exposure may cause emphysema. During the etching process, flammable hydrogen gas is also produced.”

About Dutch mordant, the CSA says: “Mixing hydrochloric acid with potassium chlorate to make Dutch mordant produces highly toxic chlorine gas. Potassium chlorate is a key ingredient in many pyrotechnics, and is a potent oxidizing agent. It can react explosively with organic compounds, sulphur compounds, sulphuric acid or even dirt or clothing. On heating it can violently decompose to oxygen and potassium chloride. Storage and use are very dangerous and require special precautions especially when mixing.”

The 19th-century practitioners of photogravure introduced ferric chloride as an etchant. Because of its unique ability to seep through a permeable gelatin resist very gradually, ferric chloride was found to be ideal for the variable depth of etching required in photogravure. It is this variable depth of etch, controlled by the variable thickness of the gelatin resist, that makes possible the variety of tones in photogravure prints. The traditional acids generated excessive heat, bubbles, and were prone to foul-biting. Ferric chloride gives a more controllable etch, plus it is safer than the traditional acids. According to the CSA: "A safer substitute for etching copper plates is ferric chloride (iron perchloride). This forms acidic solutions so should be handled accordingly, but does not have the dangers of handling concentrated acids. Ferric chloride solution might cause minor skin irritation from prolonged contact."

Ferric chloride is actually a salt, not an acid. It is usually supplied in liquid form at 45- or 46-Baumé (a measure of concentration) and can be diluted to 42-Baumé (as measured by a hydrometer) for etching use. For soft-ground etching a dilution to 32-Baumé is good. Ferric chloride is a sort of warm rust color when fresh and gradually turns dull brownish-green after much use. I usually top up used solutions with fresh ferric chloride and discard the excess rather than discarding the whole thing. This keeps a constant etching strength while conserving resources and minimizing waste. Used solution can be safely neutralized with baking soda (sodium carbonate), which precipitates out the remaining iron. Ferric chloride corrodes even 'stainless' steel, so sinks and pipes have to be plastic.

Acetic acid (mixed with salt) is indispensable for brightening copperplates just before applying resist or etching grounds or ink to them. The mixture actually forms weak hydrochloric acid which lightly etches the surface, removing oxidation. Some people use vinegar or soy sauce instead of pure acetic acid, but the impurities complicate resist laydown in photogravure. A good working solution is eight parts water to one part each of acetic acid and salt by volume.

Alkalis. Sodium hydroxide (NaOH) is useful for cleaning and degreasing copperplates. Skin and eye contact with NaOH are dangerous, but it is not volatile. Rubber gloves are essential for mixing and using sodium hydroxide. Small quantities of sodium hydroxide can safely be poured down the drain, as it is the active ingredient in drain cleaner. Ammonia, sometimes used to adjust the pH balance of solutions, “is a skin irritant and highly toxic by inhalation. Ammonia is highly corrosive to the eyes. It has good odor-warning properties [it stinks].” Non-toxic alternatives for cleaning and degreasing copperplates include: <A> a slurry of magnesium carbonate and water, <B> baking soda (sodium carbonate), <C> various vegetable oils including ordinary salad oil, <D> Basic-H household cleanser from Shaklee, and <E> vegetable esters or vegetable cleaning agents (VCAs). These substitutes require more effort to remove etching ink, but are non-volatile and safe to use.
The copper plate etched and ready to be inked
The Copper plate inked up and ready to be printed
Etched plate
Inked plate
The print
The print and the plate immediatedly after printing
Whilst the initial bite and lightest bites may seem extremely separated, remember that as you complete each successive tone, more and more of the plate is exposed, effectively accelerating the action of the acid.
I always think of acid as working like a group of hungry kids being let loose on an empty sweet shop; swarming around the brightest and stickiest sweets available and devouring them ravenously until too full to move!
ALL SAINTS STREET - Hastings, East Sussex , East Sussex - Etching by Colin Bailey
The Press
1 Age of the acid - Fresh acid will have a relatively aggressive initial phase. Older acid will have a more sustained but slower bite.
2 Room temperature - Acid reacts quicker the warmer it is and in doing so will heat up even more. Allow anything up to 25% longer for cold acid.
3 Area of metal exposed - An evenly distributed and elaborate drawing will bite quicker and more evenly than a drawing with heavily worked areas and large unexposed areas; the acid will also be "attracted" to the heavily worked areas in preference to individual lines or details..
4 How long the plate has been worked on - No-one works in a sterile environment and so the older exposed lines will have been in contact with the air for longer. They will have oxidised or have attracted grease from the air or hand. This will mean recently drawn areas will bite quicker and deeper. A solution of vinegar and salt carefully dabbed over the plate with a cotton wool ball will freshen the older lines somewhat.
A formula for achieving an set number of evenly spaced tones:
I used to have a spreadsheet which I am currently trying to resurrect which worked out the exact timings for an adjustable number of tones. I will make it available eventually! (I am presently trying to convert it from my old Psion 3a to Excel.... aaargh!!)

In the meantime here is the timing I generally use. In principal it adopts the same mathematical progression as camera shutter speeds or f stops.
1 Initial sketch with simple tones
SOFT GROUND, Dutch Mordant on Copper
30 minutes and remove ground

2 Secondary drawing with shadows and more detailed mid tones :
SOFT GROUND, Dutch Mordant on Copper -
1 hour and remove ground

Apply coating of hard ground, smoke and do not remove between bites.

Dutch Mordant on Copper working from darkest black downwards:

1 First Bite Black - 2 Hr (Total 4 Hr)
2 Then: Darker shadows - 1 hr (Total 2 Hr)
3 Shadows - 30 min (Total 1 Hr)
4 Mid - 15 min (Total 30 min)
5 Light - 8 min (Total 15 min)
6 Light 4 min (Total 8 min)
7 Lightest 4 min (Total 4 min)
8 White

Giving a total of 8 tones including White with a total of 4 Hours biting time
Each tone is double/half the time of the next
Hatching can be "interlaced" to create intermediate tones
1 2 3 4
The scariest part of etching is undoubtedly the acid! Endless films featuring bubbling, steaming bottles and beakers brandished by babbling mad scientists has given acid a bad image. YES! it is dangerous and should definitely be handled with care, especially at the mixing stage. NEVER add water to acid to dilute it - it will heat rapidly, probably spit and possibly explode. ALWAYS dilute acid by adding it slowly to cold water and ALWAYS in a well ventilated room with running water at hand in case of spills.
Nitric acid, which when diluted 1 part acid to 7 parts water gives a perfectly adequate bath for either copper or zinc ( do not use for both as the fumes can be dangerous) is a fast working solution, ideal for beginners, classes and experimental work. It bites vigorously and aggressively and can quickly lose its potency. It also tends to undercut and move sideways making close fine lines and hatching difficult to bite deeply. It tires quickly and timing can be difficult.
Dutch Mordant is an ideal mixture for Copper and can be used for zinc. It bites evenly and slowly, straight down and is very controllable. It slowly turns a bright turquoise with successive use and this can be accurately used to gauge its age and therefore strength.
Ferric Chloride is also used for copper. This bizarre solution is, I believe, more of a salt than an acid (I'm actually not too hot on the chemistry of all this!). Looking suspiciously like Worcester sauce it corrodes the plate, leaving a sediment which can impede its action on fine lines unless the plate is suspended upside down in the solution. It will stain anything it comes in contact with a rusty yellow and a few unnoticed spills can reduce anything metal to a crumbly biscuit texture in a frighteningly short time (I have lost a metal bath this way!)
This is the area where the experience bit kicks in. How long should you leave a plate in the acid? The only real way of finding out whether a plate has been properly bitten is actually to clean it off and print it! I have seen many students ruin days of elaborate drawing by removing the plates too early and discovering their etching is a mere spidery faint ghost of what they wanted, or too late and finding that the subtly rendered tones they were hoping for have merged into one muddy, turgid black mess. Over-biting a plate will not just simply make it darker; in some cases fine, close hatching will merge and the resulting open area will have no texture to hold the ink - resulting in pale dusty looking "bald" areas with hard black edges.
If this happens then one solution is to reground the plate and then re-hatch the area that is too dark - biting it again but to only half of the previous bite. The fragility of the resulting texture means that the area can be burnished to  a fairly light grey and subsequently redrawn.
A traditional etching press
With etching having been around for such a ling time  it is not surprising that a good many books have been written on the subject - the best by practicising printmakers.

The Art of Etching by E S Lumsden is the book I used when first learning etching and I still have that original battered and acid stained copy! This is the best technical guide to etching I have found and contains sate by state examples of the author’s own etching as well as a good selection of the best known Etchers
Ryepress resources is an Amazon “Astore” in which I have gathered together a selection some of the Art Materials and equipment that I have used personally and can therefore recommend.