Image Structure

Letterpress halftone prints are most easily identified by their characteristic ‘ink squeeze out,’ also called ink squash, which can be seen under magnification. This identifier is created when the inked printing matrix is pushed into the paper’s surface during printing. The pressure from the printing press forces a build-up of ink around the dot edge. This halo of ink is darker than the center area of the dot. Ink squeeze out is usually easiest to see in areas of light to medium tonality. A rough paper surface combined with low printing pressure can make the ink squeeze-out more difficult to see.

Another key identifying feature is that the image is composed of a halftone pattern; a regular network of large, sharp-edged dots. The halftone process involves the use of a process camera, which captures an image, projected through an angled halftone screen, on a sheet of high-contrast film. This patterned film is used to photographically create a metal relief plate that can be used for printing. This step is repeated using different screen angles when multicolored prints are desired.There are two types of screens. Amplitude modulated (AM) screens are the most commonly used screens in letterpress. AM screen dots are different sizes and evenly spaced. For example dots are larger in the shadows and mid-tones than in the highlights. Four color prints with an AM screen typically have a rosette pattern which is formed because each color separation is given a screen made at a different angle, each of which is specifically calculated to avoid moiré patterns. Less commonly used in letterpress is the frequency modulated (FM) screen, which produces a halftone with regularly sized dots printed at different frequencies. For example there are more dots in the shadows than in the mid-tones or highlights. Regardless of the type of screen used, the dots can be various shapes including round, elliptical, and square.

Layer Structure

Letterpress halftone prints can have a single, double, or triple layer support depending on the structure of the paper used as a support. Single layer prints consisting of ink on uncoated paper are less common than two layer and three layer prints consisting of ink on coated paper.

Deterioration Characteristics

High temperature, inappropriate relative humidity, and contact with poor quality storage or framing materials will cause discoloration and embrittlement, which leads to cracking of the paper support and/or surface coatings and loss. Poor handling may also cause abrasions of the printed image and cracking.