The Edinburgh Etch: A Breakthrough in Non-toxic Mordants
All of the information on this page is attributed to Keith Howard's book "Non-toxic intaglio Printmaking".
This text is a copy of the information from this website found here: https://www.polymetaal.nl/
The Edinburgh Etch made its official debut in Printmaking Today Vol.6, No.3. in 1997. Friedhard Kiekeben, artist and researcher at the Edinburgh Printmakers Workshop and inventor of the Edinburgh Etch, has kindly provided the following updated information.
The New Etching Chemistry
In the acrylic resist etching system metal plates are etched in solutions based on ferric chloride. This mordant which is a saturated solution of corrosive salt crystals has, over the centuries, been valued by etchers as the most accurately biting and controllable acid.
From a modern perspective it commends itself also because it gives off no toxic vapours, neither by itself nor during etching, causes little hazard during occasional skin contact, and cannot lead to violent reactions if it is accidentally spilled. The list of benefits almost looks too good to be true. But if etching with ferric gives the better etch and is so much safer than other methods why is it then that it has not always been used as the prevalent mordant in etching?
The answer simply lies in the speed and ease of biting. The way ferric was used in the past meant that etches of a reasonable depth, as are typically required in the intaglio medium, took a very long time to accomplish, and the methods employed often were cumbersome and inefficient.
However a number of ground-breaking innovations in working with ferric have now become available to intaglio printmaking which dramatically enhance the properties and the biting speed of this safe mordant; some of these have long been in practice in industry while others have resulted from my research into innovative mordants at the EPW. Due to technical limitations the use of ferric chloride has in the past been mainly restricted to etching copper plates. With the introduction of the Edinburgh etch method, ferric based mordants are now capable of eroding any kind of metal plate suitable for intaglio printmaking. These encompass not only the most common metals copper zinc, and mild steel, but also aluminum and brass. In many cases the straightforward tray-etching method known from other acids suffices, while it is feasible for larger professional workshops to also provide so-called dip-tank facilities that can further enhance the biting process on copper and steel plates.
The problems traditionally associated with ferric chloride are to do with its peculiar biting chemistry: when metal plates are etched in ferric chloride normally a sediment is gradually building up inside the bitten intagho areas. If these crystalline residues are not continually removed by some means they eventually clog up the newly formed grooves and prevent the plate from etching any further. One way of avoiding this problem is by etching the plates in so-called dip-tanks - a reliable technology from the electronics industry which has first been introduced to printmaking by the Canadian/Australian Keith Howard. The plates are inserted vertically in the corrosive solution so that any sediment particles can drop out of the etched grooves and sink to the bottom of the tank. A dip-tank should be equipped with an aeration facility which continually pumps air down to the bottom of the tank. This device agitates the ferric and makes it continually flow past the plate surface. Also the dip tank can be heated by simple means to accelerate the etch further. Especially copper plates can be etched with unrivaled speed and quality by using a tailored Edinburgh etch solution and dip-tank technology. Dip-tanks are also advisable for biting steel, but should not be employed for the more reactive zinc and aluminum which are always etched in trays.