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 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!)
Biting Times
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 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!)
Biting Times
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. Overbiting 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.
How quickly, and deeply the acid bites is a matrix of several different variables:
Age of the acid - Fresh acid will have a relatively aggressive initial phase. Older acid will have a more sustained but slower bite
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.
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.
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 a spreadsheet on my old Psion3a 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.
1a Initial sketch with simple tones ; SOFT GROUND, Dutch Mordant on Copper - 30 minutes and remove ground
2aSecondary 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.
HARD GROUND, Dutch Mordant on Copper working from darkest black downwards:
1b First Bite Black - 2 Hr (Total 4 Hr)
2b Then: Darker shadows - 1 hr (Total 2 Hr)
3b Shadows - 30 min (Total 1 Hr)
4b Mid - 15 min (Total 30 min)
5b Light - 8 min (Total 15 min)
6b Light 4 min (Total 8 min)
7b Lightest 4 min (Total 4 min)
8b 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
Whilst the initial bite and lightest bites may seem extremely separated, remember that as you complete each succesive 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 ravernously until too full to move!
