This blog article was written by Zoe Voice, Preventive Conservator.
Transparent Papers, particularly those found in archives, are a material that can look quite intimidating to handle due to their often-poor condition and brittleness. However, the plans drawn on the transparent papers in our collection, which illustrate major public works over the 19th and 20th centuries, are also a rich source of historical information, recording how the built environment around us changed through industrialisation and the modernising of infrastructure, and are a key point of interest to our researchers. Indeed, if you live in the UK, it is almost certain that you would be able to find a plan that involves a city, town or village near you, and these records are frequently requested. The Parliamentary Archives concurrently has two projects underway concerning the Deposited Plans Collection: a survey being conducted on the contents and condition, around 30,000 records, a proportion of which are on transparent paper, and a pilot treatment project by the Collections Care team to hone how we approach the conservation of transparent papers that are in poor condition. Transparent papers are a fascinating example of how the unique properties of paper fibres, made up of cellulose, can be manipulated through ingenious processes to create desirable working characteristics. In this first half we will explore the materiality of these documents.
What is Transparent Paper and how is it made?
There are several types of these papers, made using different processes that achieve the transparent effect.
The type that is most prevalent in the collection of transparent papers held by the Parliamentary Archives is Prepared, or Impregnated, tracing papers, due to their prevalence and popularity when drawing up engineering and architectural plans. These are visually obvious to identify, as aged impregnated papers have a dark yellow-brown colour, which is shown in the pictured example below. Prepared papers are produced by treating a sheet of paper which has already been made, in contrast to other methods which treat the fibres before the sheets are cast. To understand how prepared papers are made then, we must first briefly look at some key characteristics of paper itself.
Cellulose, the primary component of paper, is a structural part of plant cells, supporting them and making them strong under the pressure from the water inside the cell. Paper is manufactured by taking plant matter such as wood, hemp, flax (linen), bamboo, mulberry tree bark, esparto grass or cotton, and mechanically beating them and/or chemically purifying the pulp to remove unwanted material for a high enough cellulose content to make a thin, strong sheet of material. Cellulose molecules are attracted to each other, bonding to form long chains called polymers. These long chains are thence attracted to each other, making microfibrils, which are also attracted together, forming larger, longer fibres. If you look at paper under a microscope, you will see that it is a dimensional, complex mesh of interwoven fibres, almost like a huge tray of spaghetti.
An important feature of cellulose is that it is also transparent. You will know though that a normal piece of paper is not transparent – why is that? Going back to the spaghetti image, you can imagine that a sheet of paper contains many air gaps within these long thin pasta fibres. Because paper is a rough, irregular surface, when light hits it, the rays are scattered and diffused, making the paper look like a matte, solid object to our eyes.
However, since the 15th century the practice of impregnating a paper sheet with oils and resins to make the paper become transparent has been used. Oils and certain resins have a similar refractive index to cellulose, meaning that if all these air gaps in paper are filled with oils, the sauce in this analogy, the substrate is smooth and dense, allowing more light is transmitted through the paper, making it appear more transparent to our eyes. Which is why, for example, paper used to wrap up your fish and chips becomes greyish/clear where the oil soaks through. Below is an image of a piece of normal printer paper with droplets of water and linseed oil, which is used as an impregnation in transparent paper manufacture. Note how much clearer the pattern underneath the paper is in the area where the linseed oil droplet is.
It is also possible to combine impregnation with other methods of production, such as Imitation Parchment Paper or Natural Tracing Papers. These are processes where the fibres are treated before being cast into sheets of paper. For both types, the pulp slurry is beaten for much longer, until the cellulose fibres are flattened and stretched and thinned out so that they can coil tighter together when cast into a sheet. This again means that there is less space for air gaps, making light able to transmit more effectively. More like angel’s hair pasta than bigoli pasta when we’re imagining spaghetti fibres. These look transparent on their own, or an impregnation is then also added for extra effect. This is a much later invention, in the late 1870s, as this required paper pulp beaters which specifically were designed to stretch and flatten the fibres in this way.
There are more types, all of which have their own unique processes and histories, but for the sake of brevity we’ll leave those for another time. Transparent papers in the Parliamentary Archives are a record of industrialisation in and of itself, as well as holding written records of major public works. The development of new types of transparent papers being produced, or variances in their processes, are borne out of the advances being made constantly in the 18th and 19th centuries in the mechanisation of paper making to respond to increasing demand for paper.
What types of items are on transparent papers in the Parliamentary Archives?
Permission to make changes to the UK’s infrastructure, such as building roads, railways, canals, and other major works has, since the Medieval Period, had to be granted by Parliament. From 1794, it became part of the process of application to submit a plan of the works, which would be drawn up in a scaled plan. Their formats and styles vary - some are small, hand-drawn and coloured, some are printed and come in a pamphlet style, some examples in our collection are over 8 meters long! They would also often also come with associated records such as an estimate of expense, and book of reference, and sections (cross-section drawings) of the proposed works. There is a guide on searching through these records on our website. A particularly colourful example that came through our studio was a plan that covered where the Olympic Park in Stratford now sits.
What condition challenges do they present?
Paper is an organic material, so it decays over time. As paper ages, it becomes increasingly brittle – though the rate of this depends on the quality of paper and the conditions in which it is stored. This is because cellulose chains degrade by depolymerising, where these long, flexible chains are successively broken down into smaller and smaller segments until the sheet itself is notably weaker and discoloured. These breakages occur in the amorphous regions of the cellulose chain, which also give paper its flexibility, so as the paper degrades, fibres get shorter and less flexible. Oxidation and Hydrolysis are the two main mechanisms that break down the chains. Oxidation is where oxygen reacts with hydroxyl group cleaving the cellulose molecule and producing carboxylic acid. Hydrolysis is where a molecule of water reacts with cellulose, again breaking the chain. These processes will occur at an increasing rate throughout a paper’s lifetime, but improper conditions of storage, i.e. significant light exposure or damp conditions, accelerate both of these rates. Further, the presence of acids, as is inherent in low-quality wood pulp paper like newsprint, or if the paper’s fibres were quite short to begin with because it was made using large industrial beaters and cheap pulp in contrast to handmade cotton rag paper, will speed these reactions up exponentially. This is why you might be see newpaper clippings or paperbacks from only a few decades ago look dark and feel fragile, whereas an expensive antiquarian book from over 300 years ago looks like it was made yesterday.
However, tracing paper is also especially susceptible to mechanical damage - this is for a number of reasons. Firstly, as we discussed above, the paper substrate is dense without many air gaps, so it is thus very brittle, as there is not a lot of room within the sheet for the fibres to flex around as the paper is moved. Secondly, tracing paper plans are often very large, meaning that in the past they must have been awkward to handle and store without being squished, folded or bumped. Thirdly, these documents are working, information-rich items, rather than precious fine art objects which are framed and hung on a wall, so their wear is a testament to their life of use. You can imagine these plans being drawn, rolled up, carried to a building site, unrolled, then returned and put into a drawer in an office, then sent to Parliament where they are further studied before finally being put into the Parliamentary Archives. This life of use means that tears, losses and areas of snapping along fold lines are very common.
Other damages may come from the environmental conditions in which the records were stored, i.e. areas that are damp or warm, however as our records were deposited into the Archive repositories relatively quickly these damages are less common in our cases.
In the next post, we’ll take a look at a few examples of transparent papers currently undergoing treatment in our studio, and also some we have come across which were treated historically by the collections care department at the Parliamentary Archives
 Transparent papers are also found in many other types of collections such as those held by galleries, as they were used by artists for their beautiful translucency, for example. However, this blog post will focus on transparent papers that are most often used and engaged with by the public in an archive setting.