In the Collection Care Department we are responsible for preserving the documents stored at The National Archives and ensuring that the public can access them. Decades ago, in an effort to meet these goals, conservators at The National Archives laminated many commonly accessed documents in the collection using semi-transparent tissues.
These ‘area bonded fibre laminates (ABF)’, are covered on one side with a thin layer of adhesive (usually a synthetic acrylic polymer). During lamination, a document was sandwiched between two ABF tissues and pressed using a heat press at approximately 85°C for one minute. In the 1970s and 1980s, this process was considered state-of-the-art in ensuring document preservation.
Although the ABF laminate tissues and synthetic adhesives used on many of our documents are in relatively good condition, times have changed and lamination is now not only considered unnecessary, but in some cases, unsightly.
In addition, the long-term effects of the adhesives have not been fully explored, nor could the consequences of lamination be known or studied for every type of paper and ink in our collection. So earlier this year, conservator, Ilaria Budgen, and I started exploring strategies for safely removing the lamination. We created model samples for testing using de-accessioned documents written on a variety of paper types, with text in iron gall ink, pencil, wax pencil, ball point inks, printer inks, and stamp inks. The models were artificially aged using heat and high relative humidity to accelerate some characteristic deterioration processes.
Most contemporary conservation treatments should be reversible, or at least ‘re-treatable’. In the brochures for ABF laminate tissues, you can often spot a statement like ‘spirit reversible’, which may lead to a false sense of comfort. But what does ‘spirit reversible’ mean in the context of a document fully encapsulated in laminate? Should we bathe the document in spirits to dissolve away the adhesive? Which type of ‘spirit’? This term is used both for ‘white spirit’ and ‘rectified spirit’ – essentially paint thinner and alcohol, respectively.
Critically, since the documents are already laminated, we cannot check if any of their inks are sensitive to either type of spirit. Without this knowledge, we risk losing the writing by attempting to reverse the lamination with a ‘spirit’ wash.
We first tried reversing the lamination using heat. Since the ABF tissues were applied using a heat press to activate the adhesive, we thought we might be able to use heat to reactivate it and peel off the laminate.
To achieve this in controlled way we used a prototype iMAT on loan to us from the Munch Museum in Oslo, Norway. The iMAT is a transparent mat containing carbon nanotubes and silver nanoparticles, which can heat up in a precise and controlled matter at a rate of 0.1°C.
We attempted to remove the laminate from our model samples after they had been pressed under the heated iMAT at different temperatures and for different time periods. Unfortunately, we discovered that the adhesive sticks more strongly to the paper than to the semi-transparent ABF tissue. Once reheated, the ABF tissue easily peels off, leaving a layer of adhesive on the paper beneath!
Over several months of testing, Ilaria developed a method for removing the ABF laminate and the majority of the accompanying adhesive using acetone vapour delivered through a microporous tissue.
This technique greatly reduces the risk of dissolving away the inks, and we can test in a small area of the document first, to ensure that the inks are not affected.
Now Ilaria and I are working on a series of experiments to measure whether all of the adhesive is being removed by her treatment method. We’ve also started testing whether the lamination has beneficial properties for the documents, like protecting some inks from fading over time. Stay tuned for the results of these studies!