Copper in Aqua Fortis: Etching a Plate

Everyone loves a little movie, and everyone loves bubbles, right?

This is the easiest (and most interesting) way to show the process of ‘biting’, or etching, a copper plate for intaglio printing. It’s a short video made whilst one of my plates was being bitten – the copper turns the acid blue, hence the strange colour, and the ventilation system was on, hence the noise. The floating particles on the surface of the acid are mostly disintegrating feather, explained below.



As described in more detail here, the plate is prepared by being covered with an acid-resistant ground (or ‘resist’), and an image drawn through the ground with a scribe or etching needle, exposing the copper beneath. The plate is then carefully lowered into a tray (in this instance, chemical grade plastic) filled with nitric acid – Aqua Fortis – diluted by water, usually at a strength of 1:4 or 1:8.

As the acid reacts with the exposed copper, bubbles of gas form, which collect on the surface of the plate. Some release from the surface – and you can see them popping on the surface of the acid – but many do not. These need to be disturbed every now and then by using a feather to stroke the surface of the plate, which can be colloquially known as ‘feathering’. The feather does not damage the ground/resist, but it is, of course, slowly dissolved by the acid.

Removing the bubbles periodically ensures that the lines bite evenly. If the bubbles are left on the plate, the exposed metal underneath/inside the bubble is not bitten at the same rate as other exposed areas. There can even be an uneven texture left behind which indicates areas where the acid bit around, but not inside, the bubble, which can be evident on the final print. This can be seen in the detail of the image below.


Smaller Frans Crabbe Esther Before Ahasuerus etching 1520s  Detail of Frans Crabbe Esther Before Ahasuerus etching 1520s
Above left: the whole image and right: a detail from the top left corner, showing an area which has not bitten properly due to the presence of bubbles

Frans Crabbe
Esther before Ahasuerus


When it is time for the plate to be removed (the length of which depends on the plate, the image, and the desired effect), the plate is removed by using an inert object made from wood or plastic. Interestingly enough, I recently had to get a plate out of the acid and discovered that someone had broken the plastic cake slice that is usually kept for the purpose. After looking around I finally had to use a metal palette knife. In the couple of seconds that it was placed in the acid and in contact with the copper plate, the knife was anodised with a thin layer of copper. Very pretty indeed!

Nitric acid has been a popular choice for etching metals from the sixteenth century up into modern practice. It is more complicated to produce than the simplest way of etching – using a combination of salt and vinegar – but is much faster. When one considers that it can take up to four weeks to etch a line about 0.1mm deep into a copper plate using common salt and vinegar (or several hours by using a more complex salt and boiling the vinegar), and approximately 10 – 15mins for the same result with a solution of nitric acid, the advantage can be clearly seen in the production of an intaglio matrix. According to Ad Stijnman in Engraving and Etching, nitric acid was being produced on a regular basis and was readily available from the fifteenth century in Venice. This was primarily as a result of saltpetre (the source of nitrogen in nitric acid) being manufactured in quantity for firearms.

Images of the process prior to the end of the sixteenth century seem to be nigh impossible to find, and the best at this point in time is the one below, from the first treatise on the subject, Manual of Etching by Abraham Bosse, first published in 1645.


Smaller abraham-bosse-1644-maniere-de-jetter-leau-forte-sur-la-planche
Abraham Bosse
Maniere de jetter leau forte sur la Planche
France, 1645


As an interesting aside, I put the text from the image, “maniere de jetter leau forte sur la planche” through an online translator and was given the result “way to throw water on the high board”. However, when I placed an apostrophe into “leau” to become “l’eau” the result changed – “maniere de jetter l’eau forte sur la planche” became “way to throw the etching on the board”. It would seem that the concept of l’eau forte has become entrenched with the process of etching. In as much as an online translator will tell me, of course.

Here we see that that the plate is not immersed, but the acid poured onto the plate and collected at the bottom. This would, presumably, take longer than immersion to etch a line. One advantage of pouring that I immediately thought of would be that bubbles would not have the chance to form, and potentially distort the line. As far as my research has taken me, I haven’t yet seen when this change would have taken place. From a modern health and safety point of view, however, I am more than happy to not have to be continually pouring acid, and prefer to use the immersion method.

And to be honest, I also find feathering, and watching the bubbles form, swell and pop, to be somewhat mesmerising, and one of the nicest things about etching (acid fumes aside).