Biting with Acid in Etching

Biting and Acid in Etching

In etching, the plate’s topography, the incisions or pits that later hold the ink during printing, is created through chemical rather than mechanical action. To ‘bite’ lines into the metal, the plate, previously covered by a layer of resist, is placed in a bath of a dilute inorganic acid, such as Nitric Acid or Ferric Chloride. In biting, controlled operations of drawing give way to the blind force of a chemical reaction, which constantly threatens to undermine the integrity of the plate and incised lines.

Human agency is relatively limited in the biting process. The Maker places the plate in the bath, disturbs it slightly during the biting to dislodge any bubbles that have formed, and later extracts it from the bath after a pre-determined time based on the acid strength. The process removes the object from the Maker’s hands, ceding it to the solution. The action registered on the plate is inflicted only by the acid’s chemical activity.

The biting process employs differences in solubility to direct these reactions. The acid is water soluble, while the resist is oil soluble. The hydrophobic resist impedes the acid from reacting with the underlying metal. The acid attacks any unprotected metal, such as the lines exposed by a Maker’s scratching. The acid reads a binary – exposed or non-exposed metal – and indiscriminately assails any bare plate. The acid exposes accidental marks, minute gaps in the resist, or inadvertent oily fingerprints invisible to the Maker's eye. Biting surrenders the plate to the chance marks earlier steps embedded in the resist. The acid takes all such openings literally and blindly -- it does not read intent, degree or nuance. Gradation in the depth of the lines or pits can be inserted only by repeating iterations of the biting process, varying timing and/or the resist’s application, and selectively exposing or re-exposing metal to the acid’s aggression.

Anywhere acid meets metal a chemical reaction occurs. For example, when a Zinc plate is placed in a Nitric Acid solution, the Nitrate ions react with the exposed Zinc molecules to produce Zinc Nitrate and Hydrogen gas. The Hydrogen produced by the chemical reaction between acid and Zinc forms the bubbles seen in the video. The bubbles are an indicator and necessary result of the process. These bubbles must be removed to assure the Zinc is continually exposed to the acid. Bubbles left in place can cause uneven biting, disturbing the final quality of the printed line. As seen in the video, the bubbles are brushed away with soft feathers so as not to disturb the resist. During biting, the Maker continually removes one product of the chemical process to perpetuate the reaction. The Maker maintains the acid’s agency by removing the Hydrogen gas byproduct, assuming the role of waste manager while the acid marks the plate, fixing the image's form.

– Thea Goldring, PhD Student, Harvard University