The Salt-Sulphate Etching for Aluminium.

Due to its softness and coarser structure, aluminum is slightly less suitable for intaglio printing than other metals. Nowadays, aluminum is used for dry needles and as a support for photopolymer film. 

However, the salt-sulphate etching gives special effects on aluminium. A clear surface roughness arises during the etching of aluminum with the salt-sulphate solution. This structure is comparable to "mezzo-tint" or "aquatint". If a plate is printed with this "natural" roughness, a deep black is created. This means that an "open bite" (crevé) is not possible with this technique. This gives additional options.

Normally, both copper sulfate and salt have no direct corrosive effect on the metal. Etching is only possible if both substances are dissolved, whereby the amount of salt (sodium chloride) must be at least twice the amount of copper sulfate. Other metals can be etched easily, as long as they are free of grease. This does not apply to aluminum. Aluminum forms a dense oxide skin very quickly. Therefore, just before etching, the aluminum surface should be sanded with fine steel wool. This should also be done before any acrylic etching ground is applied. 

As with zinc etching, a coppery "sediment" is formed which floats to the surface and needs to be removed regularly. Bubbles of hydrogen gas also appear on the surface during etching. 

The Salt-Sulphate Etching for Aluminium. 

1. 70 grams of copper sulfate

2. 140 grams sodium chloride (table salt)

3 1 liter of water

Mix larger quantities in the same ratio.