Do you feel you can pretty much recognise most printing techniques when looking at artworks on paper? Or have you sometimes wondered whether you are in front of an historical print or a very good copy of an older technique? Is it a genuine print, a ‘fake’ one or an artistic interpretation of various techniques?

These very questions were what ‘Identification of Photomechanical Prints’, held in the conservation studio of the Wellcome Collection on 8-9 June 2017, aimed to address.

The course was organised by International Academic Projects (IAP), in collaboration with conservators at Wellcome, and Hildegard Homburger was the course tutor for the two days. The nine of us attending the course (conservators in private practice, and from national and international institutions) enjoyed exposure to Hildegard’s vast knowledge of the huge variety of commonly encountered printing techniques.

Hildegard’s lectures were followed by practical sessions in which the various techniques were first clearly explained via PowerPoint presentations and videos, and then observed in prints under the microscope.

The course focused on photomechanical printing processes, but also touched on manual ones where it was important to compare techniques. We spent the last part of the course split into two groups, observing examples of approximately 20 different printing techniques under a microscope and trying to name each one, keeping in mind their characteristics and distinguishing features as taught by Hildegard.

It was mind-blowing to realise how many kinds of techniques exist and how very subtle the differences between them can be. We were made aware of how important it is to train our eyes and look at as many prints as possible. It is also essential to pay attention to many details (and not only the obvious ones), for example looking with a raking light at the verso of the print, finding a possible relief image, etc., and to consider the context of the actual work before attempting to identify the printing technique used by the artist. Hildegard had the most generous amount of prints of each kind of technique for us to look at and to compare with very different and/or similar but deceiving ones. The practical sessions were a veritable feast for the eye, and for our curiosity, as we wanted to learn as much as possible from Hildegard in the two days we spent with her. A real treat!

Photomechanical Processes

Hildegard listed the photomechanical types of printing as follows: relief, planograph, stencil, intaglio, and non-impact print. For each type, we were given useful tips on how to recognise its distinguishing features.

RELIEF

General distinguishing features:

• In the matrix, the ink is pulled from the raised parts of the plate

• In the printed image, the ink is pressed into the paper; the ink is squeezed to the rim; relief might be noticeable on the verso in raking light

Techniques:

• Line block

• Halftone relief (with crossline screen, with grained screen)

• Hybrid (black key plate in halftone relief, colours in manual planographic processes)

PLANOGRAPH

General distinguishing features:

• In the matrix, the inked areas are on the same level as non-inked areas

• In the printed image, the ink is even and flat on the paper surface; no relief is visible

Techniques:

• Planograph without halftone, in which the grain of stone or metal can be visible

• Halftone planograph, with crossline screen or amplitude modulation (varying sizes of dots which have no sharp edges) and with frequency modulation (with distribution of very small dots)

• Photochrome, with amorphous grain pattern; is often manually worked over

STENCIL

General distinguishing features:

• In the matrix, the ink passes through open areas of stencil (woven screen)

• In the printed image, the ink sits on top of the paper, and traces of weave pattern are visible.

Techniques:

• Screenprint without halftone, which could be mistaken for the manual screenprint

• Screenprint with halftone, in which the screen pattern is mostly visible without magnification and there are traces of weave pattern along the edges of the dots

INTAGLIO

General distinguishing features:

• In the matrix, the ink is pulled from the grooves into the plate

• In the printed image, the ink has a relief on the paper surface

Subsections of the intaglio process are the photogravure and the rotogravure.

Photogravure techniques:

• Line image, with mostly aquatint grain; the lines with grain appear less sharp; sometimes with manual corrections; the plate mark is usually larger than the image size

• Halftone image with aquatint grain, in which the grain is visible as light dots and the plate mark is usually larger than the image size

• Halftone image with grained screen, with grain pattern of amorphous dots; the plate mark is usually larger than the image size

• Polymer photogravure with aquatint screens, with various grains, and plate marks without distance to the edge of the image

Rotogravure techniques:

• Rotogravure with crossline screen, with no plate mark; grid of light lines with rhombical dots of the same size; the letters have regular broken edges

• Rotogravure with grained screen, with no plate mark and grain pattern of round(ish) dots

• Cylinder gravure, of which off-square dots can be of different size with a certain uneven thickness of the ink

NON-IMPACT PRINT

General distinguishing features:

• In the matrix, data sent from the computer directly to the printer

• In the printed image, the powdered or liquid ink is melted onto the paper surface (laser print), or ink droplets on paper (inkjet)

Types of laser prints are:

• Dry toner, which has mostly amplitude modulation and powdered particles around the dots or lines

• Liquid toner, also with amplitude modulation; it has round dots and very small toner dots

Inkjet prints have frequency modulation and very small dots either sunk into the paper or on the paper surface.

Hildegard gave each course participant a poster with a summary of all of the mechanical techniques she taught us. A very useful tool to keep at the bench, it is a checklist and reminder of the distinguishing features to look for when observing and identifying prints.

I would like to thank Hildegard for sharing with us her extensive knowledge on photomechanical prints, for her patience and for making learning with her such fun.

All photographs courtesy of Hildegard Homburger.

Stefania Signorello ACR received both a Diploma and a Degree in Fine Arts before moving to Florence in 1991 to study Book and Paper Conservation. She gained work experience in Rome, Florence, Milan and Prague before moving to London in 1997. She has since worked at a number of institutions in the UK, including the Victoria and Albert Museum, the Theatre Museum, the Bodleian Library and the Oxford University Archives. Stefania has been an accredited member of the Icon Book and Paper (B&P) Group since 2001 and has served as both Deputy Chair and Chair for the group’s managing committee. She has been working as a conservator at the Wellcome Library since 2003.