Saline Sulphate Etching

Is a rediscovered etching mordant that was employed prior to the mass production of nitric acid during the industrial revolution. Unlike nitric acid the saline sulphate mordant does not produce noxious gasses and can be used in an open studio without the need for special ventilation systems. It has now largely replaced the use of nitric acid as a mordant in public printmaking studios. Equally important to me as an artist is the nature of the marks that can be made with the mordant in combination with different types of resists. Saline sulphate produces a very deep etch in aluminium and zinc.  Most metals the unprotected areas when etched have an open bite, a flat area with no tooth to hold the ink.  Aluminium  etches with a textured surface that holds the ink and depending on the depth of the etch, produces a light grey to a deep black when printed.

The saline sulphate etching is a  galvanic process.  All the ingredients are present to produce a galvanic cell. The electrodes consist of the two metals. The cathode is the copper sulphate, the anode is the aluminium plate and the electrolyte is the sodium chloride in solution. The copper ions from the copper sulphate create a powerful electrical potential that attracts atoms away from any exposed areas of the aluminium cathode. The etching potential is dependent on the level of copper ions in the solution; the more there are, the higher the potential. The process that produces and transmits the electric charge that facilitates the migration of the metal ions is contained within the solution of the galvanic battery or passive etching cell.  The larger the bath is relative to the size of the plates the higher the electrical charge will be, enabling more etching to take place over a longer period. 

This print is made up of five separate aluminium plates. The images on the plates were drawn with a permanent marker that acts as a resist to the saline sulphate that was used as a mordant to etch them. The breaks in the etched lines are where the resist has broken down providing interesting marks that bring the plate to life. The plates are very deeply etched providing a heavy black in the intaglio and a lighter grey where the permanent marker has broken down. The depth of the intaglio provides a higher surface that can be relief rolled with a different colour.The advantages that saline sulphate has over traditional acid etching is that it does not produce toxic fumes, but it does release hydrogen gas that is potentially explosive so requires a well-ventilated environment. It shares some of the disadvantages of traditional acid; it requires monitoring because timings need altering as the mordant weakens. The disposal of the mordant is also an issue. It provides a deep etch in aluminium and zinc, but lines can be ragged and run into each other if they are too close. Care has to be taken when open biting because the natural aquatint can disappear leaving a flat surface if the plate is over etched. The deep etch that this mordant can give on both aluminium and zinc plates provides an excellent surface for viscosity printing.
Resources
Adam, R and Robertson C. (2007) Intaglio: the complete safety-first system for creative printmaking, London: Thames and Hudson. This is pretty much the definitive step by step guide to  safe etching. 
Nontoxic Printmaking, Safe Painting & Printed Art – nontoxicprint This is the ‘go to site’ for advice and instructions on less toxic and safer art making. The link will take you straight to the best step by step guide available on saline sulphate etching.