Introduction
Transparent media has been used throughout the centuries by artists and craftsmen to assist in the creative process. Transparent papers specifically begin to show up in artist recipe books almost as soon as paper itself appears in Europe. Despite that, the literature on the subject is field-specific and challenging to access. The following paper serves as an abridged retelling of the information currently available on the subject on transparent papers. It is not comprehensive, partly due to some literature being outside of the author’s access, and partially due to constraints in time. This paper seeks to look at the available literature and give an overview of the scope of information available on transparent paper history and creation. It does not look into specific conservation treatments such as treatment recommendations or case study objects.
I. Literature
The literature on the subject of transparent papers within collections and as conservation material has only started to appear in publications for about the past 40 years. This timing coincides with literature remarks on the sudden interest taken in architectural collection materials in the 1980s.[1] This rise in interest sparked the need for more information on these materials. It is unsurprising then that most of the content on transparent papers within the field also tends to focus on architectural drawings.
The more significant in size publications that were consulted for this paper tend to be written on architectural history or architectural drawings as a whole, of which transparent papers play a role.
Smaller articles within the conservation field tend to look at a survey of transparent papers within a collection and analyze them, working from the objects towards identification. This is often due to the fact that historical literature on the creation of transparent papers is sparse. Many of the more historically driven writings can note the first mentions of creating transparent materials as early as the 15th century, often citing artist publications and explicitly citing Cennini’s artist treatise. Between Cennini and today's market, information becomes more difficult to find. Literature turns to patents and catalog entries, which can be vague or give an inaccurate view of the use at the time.[2] When not focused on surveys, conservation publications focus on treatment case studies and treatment recommendations or improvements.
Reviewing the literature on the subject, it seems that there could still be room for an overarching publication. One on transparent papers which could combine the analytical, conservation, and historical sources available while also expanding past the types of translucent documents found in collections, to those found and used more industrially.
II. What Determines Paper Translucency
The interaction between light and paper determines its opacity. The way light reflects and refracts will determine how the human eye perceives it. The way light reflects off the surface of an object is considered elastic scattering while inelastic scattering is the refraction of the light, or how it passes through one material to another.[3] A sheet of paper is made of layered cellulose fibers as well as additives that manufacturers often add to the paper-making process to gain effect on the result. When light filters through a sheet of paper, it must navigate between fibers, additives, and pockets of air. Every transition between materials causes more opportunities for light traveling through to scatter. This results in the light traveling through the material and slowly lowering in intensity in a process called ‘attenuation'.[4] Because paper is either translucent or sometimes transparent, the way light travels through the material is considered a diffuse transition of light.[5] The light will not pass through in one direction and will instead be scattered as it passes through the paper and will be directed in various directions.
Surface texture also plays a role in how light reacts to the object, determining its elastic scattering. A rough or more textured paper surface will deflect more light than a paper with a smooth surface. Obstructions or surface change on the surface of a sheet of paper can affect the transparency, even when the obstructions are minimal. The human eye can perceive surface changes on the structure upwards of ~0.07 mm. That being said, changes on surfaces smaller than 0.07 can still be visible in the form of haze, loss of surface gloss, or reduced image clarity.[6]
III. Terminology and Translucent Papers
When on the subject of translucent papers, terminology plays an essential role in the ability to identify the paper type and manufacturing process. While it is tempting to use the colloquial term of tracing papers to describe many of the papers that will be mentioned here, the name ‘tracing paper’ is one that only addresses one of the functions that some translucent papers have. The term does not give any information about the type of manufacturing of the paper in question. Within this paper, the term translucent paper or transparent paper will be used as umbrella terms for papers that have a low opacity. Typically, translucent would be a correct term as most of the papers mentioned here are not visually clear enough to be considered transparent. However, some modern papers do successfully lend more towards transparency. Additionally, one source cites that “a translucent medium allows the transport of light, while a transparent medium not only allows the transport of light but allows for image formation.”[7] Since papers used for tracing are manufactured specifically to allow image formation to be seen through the paper, and could, therefore, be considered transparent.
Within this broad range of paper types, further terminology of papers is needed. Additionally, sources on the subject are not uniform in their language at times. This section serves to list the various types and names given to transparent papers within the literature.
Several sources of literature divide their list of translucent papers into three types[8]; those made transparent through impregnation of the paper, those made transparent by mechanical means, and those made transparent by chemical treatment.[9] An explanation of those terms will be given, followed by other terms for transparent papers that have been seen throughout the literature.
Impregnated paper. Impregnated papers are those which are treated after the sheet is formed with either on or a combination of oils, resins, or waxes to create a transparent paper. Materials used to impregnate paper can vary but can include “linseed oil, walnut oil, colophany and Strasbourg turpentine, sandarac, shellac, copal resin, starch,...mineral oil, [or] synthetic resins”.[10] The additives would fill the space within the paper’s fibers so that light could filter much easier through a sheet. There are a few methods of impregnating a paper as they could be immersed, sprayed, or rolled before being allowed to dry.[12]
Mechanical treated paper. Also called natural tracing paper or imitation parchment.[13] Paper can be made transparent by over beating the paper pulp. This process can create a more fibrillated and shorter cellulose fiber, which leads to more inter-fiber bonding and both fewer and smaller pores within the paper. The shorter and more connected fibers cause there to be less space within the paper fibers for light to scatter, which in turn makes the paper less opaque.[14] Calendaring mechanically beaten papers can also increase the transparency of the final result.[15]
Chemically treated transparent papers, Acid treated transparent papers. These papers are also referred to as parchment paper, parchmented paper, or vegetable parchment.[16] The process for creating these papers is also known as sulfurization and is used on already formed papers, which are then treated with an acid which is neutralized and then washed out.[17] This process began in the 1840s but became a manufactured and ready-made mechanically produced product by the 1860s.[18] These papers were durable, greaseproof, and reliable when wet, which allowed them to not only act as printing or tracing but also for food packaging.[19]
Tissue is a term that is difficult to define as it is used colloquially for thin papers and not necessarily translucent ones. One source cites the use of tissue papers as tracing material but notes that while it was available since the late 1800s, these tissue papers were unsuitable for ink.[20] Another notes that paper is generally considered a tissue paper when it has a low paper weigh, lower than 40 g/㎡, but also notes that this differs between countries.[21]
Oiled Paper. See impregnated papers for further information. This term refers specifically to papers impregnated with oil. The literature which had this term originates from is a 1914 publication, and the publication states that this was explicitly used for paper soaked in boiled linseed oil.[22]
Waxed Papers. See impregnated papers for further information. Paper that has been passed through a bath of wax. These are usually used as food wrapping material.[23]
Onion Skin Paper. This term is used for very thin translucent papers with a high gloss, given for its resemblance to onion skins.[24] This term was only listed in one conservation article and had been sourced to a 1914 publication on paper. This term does not give any information about the manufacturing of the paper nor of its intended use and so seems to be either a more colloquial term or one from the industry of that time period.
Glassine. Noted as a predominantly Scandinavian product, the first machine made glassine is dated to 1894. “A bisulphite pulp was cooked at a low temperature with lime and then beaten for a long time.”[25] Isinglass. Also referred to as paper glacé or paper gelatine. Not actually a transparent paper but a material that was also used to create a transparent sheet was isinglass. Created "by pouring a warm solution of isinglass on to an oiled sheet of plate glass, and covering it with a similar sheet until cold."[26] Mentioned here as it can sometimes be confused with translucent papers.
IV. Diazo Types
Diazo types are a photo reproductive method used by architects to create copies of drawings. The process was invented in the 1880s, although it was not until the 1930s that they were widely used in architectural studios.[27] Because much of the literature on the subject of transparent papers focuses on architectural drawings, there is plenty of information on this particular reproduction technique as it was widely used and found in collections.[28] It is for that reason that diazo types are reviewed and explained here.
Diazo types are a ‘dry’ photo reproduction technique, which was only done on translucent support papers or plastic film supports. This was because when a diazo type is made, the image is printed in reverse, requiring the support to be translucent. Diazo types were often created with the image and ground on one side, and then read or notated on the verso.[29] These prints were used as ‘intermediates,’ meaning that they were a document created after the original drawing and would be used to make a further, final reproduction later in the process. Intermediates were often used to make corrections or notations and then reproduced to a later copy or used to reverse an image.[30]
Being a ‘dry’ reproductive process meant that the method used an emulsion rather than the solution as its photosensitive material.[31] Emulsions are gelatin-based which can be applied to paper but does not penetrate through. This also means that the paper is coated with the emulsion, making the surface smooth and covering the paper fibers.[32]
Diazo types can be identified because the image side is slightly shiny or glossy, they are printed in reverse, have brown lines, and the image is said to have a ‘dirty’ or ‘mottled’ look.[33] Over time these prints can turn yellow or brown due to the chemicals left over from the reproduction process as they oxidize over time. This degradation is often seen on the edges and the image-side of the print.[34] This can also affect items stored in contact with them as diazo types cause pink discoloration or greasy stains on nearby touching objects.[35]
V. History
The need for transparent materials has been present before paper. Transparent parchment could be created for the use of artists but also as a substitute for glass.[36] This use of parchment continued and was even patented in the 1700s.[37] This history of transparent parchment runs concurrently with that of transparent papers before the use of transparent papers began to outperform them and parchment fell out of favor .
The earliest mentions of transparent papers come from artists and recipe books from the 15th century. This paper was one method of many that artists used to transfer and trace drawings from one sheet of paper to another or from paper onto canvas. Handbooks outlining how to create transparent papers remained the primary source for information on the subject until the 18th century. It is at that time when architectural drawings became increasingly important. The 1800s saw the rise of architects as a profession, and architectural drawings were the path of legitimization for the field.[38] As the profession of architect and builder grew further apart, the necessity of these drawings and their accuracy became vital for successful communication of the design. The designs needed to be distributed to different hands and the ability to copy accurately and quickly fueled the need for new products. Thus, transparent papers became the perfect tool for draftsmen.
By 1768, tracing paper was available in a ready-made form from producers W.J. Reeves and, in 1802, was also listed by Rudolph Ackermann catalogues.[39] The shift into the 1800s also saw the rise of machine-made papers which allowed for a wider variety of papers to be made available to the public. The later 1860s saw an increase in demand for transparent papers that manufacturers readily provided.[40] By 1890, tracing papers were being used for a variety of purposes. Glassine made its appearance at this time in 1894 in Norway.[41] By the 1930s, tracing paper was being used by architects as support for presentation drawings done in dry media.[42]
As previously mentioned, diazo types were invented within the 1880s, and by the 1930s they were widely used in architectural studios.[43] By the 1950s, diazo types could be made on automatic printing machines and could produce 300 copies per minute.[44] It was also within the 1950s that synthetic resins began being used as substitutes for the traditional oils used in impregnated papers.[45] It is from the 1950s onward that information gets more difficult to find on the development of translucent papers. Modern industry creates chemically treated transparent papers for general use, creates specialty papers for food packaging, and the more general ‘tracing papers’ for graphic use.[46] More information on this end could be gained but is not yet present within conservation material. Paper industry sources are likely to be the next avenue of information for more recent transparent paper manufacturing.
Conclusion
Overall, the amount of research and publications on translucent papers has dramatically increased over the past forty years and the amount of information gathered was more than what the author had expected at the beginning of this undertaking. That being said, the scope of the topic could be improved. Many of the publications are singular articles, and the full publications are often focused on architectural works exclusively. All information on the subject is difficult to access, regularly published as field-exclusive publications, or are found in expensive and difficult to find volumes. The topic could benefit from a comprehensive book on the material subject of transparent papers, just as Price's Line, Shade and Shadow does for architectural drawings. Modern and recent practices could also be further investigated. Translucent papers are a subject of ongoing research, and so new literature is being published every year.
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