Why does coated paper stick together when it gets wet?

‘Coated paper’ refers to the smooth often shiny paper we most frequently see in glossy magazines, some books (particularly those which contain printed plates) and graphic novels.

The surface of coated paper owes its qualities to the addition of either clay, (kaolin), calcium or barium sulphate. These are mixed together with either glue or casein, both of which help to bind the clay to the paper surface. The finished paper is then calendered (or ‘polished’) to create either a matt, glossy or smooth surface which together with the minerals has a natural affinity for pigments and dyes such as those which are used during colour lithographic printing.

So why does coated paper stick together so much once it has been wetted and dried? If we recall what happens when clay dries and then water is added, what results is a substance that is sticky when wet and crumbly and hard once dry. If we factor in the addition of glue it is unsurprising that the surface of coated pages tend to stick together and set almost like a concrete block. In addition, stains embed themselves within the paper.

When separation is attempted the paper, (usually weak with thin, short fibres) can’t resist the force required and the paper then fragments with paints and inks tending to cleave at the coated surface. Wetting only encourages the glue effect to return and drying results in crumbling when separation is attempted. That often spells a big problem for the conservator!

Let’s look at an example…

Recently, two graphic novels arrived at the studio, each of which had the lower third of their pages completely stuck together after being beneath a leaking roof. Each page was set into a solid block and while fortunately the pages of one novel came apart with very little damage, the second novel was far more problematic. Typically, the pages crumbled with each attempt at separation. Eventually, the pages were separated though with considerable damage. Even the smallest of fragments were saved and re-pasted wherever possible but the lost areas of each page had to be ‘rebuilt’.

While normally in conservation the approach of ‘first do no harm’ applies, this object was neither rare nor unique and the client fully understood the consequences of separation before treatment began. Balancing the reality that the graphic novel could never be opened ever again and did not fulfil its original function, the treatment proceeded.

Whilst the pages separated, the treatment resulted in large losses. However, these losses were successfully infilled with Japanese repair papers which were selected based on their empathy with the original surface texture, weight, colour and strength of the pages. Once finished, the pages were stronger, the severe distortions flattened and the graphic novel was fully useable again.

It can never be accurately predicted how well coated paper can be separated until treatment begins but the good news is that with the application of care and the correct materials, successful results can often be achieved.

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