Scratchboard

It is very difficult to get information from manufacturers about the
make up of some art materials. The Windsor and Newton Laboratories
in the UK said that to the best of their knowledge scratchboard
("scraperboard" in Europe) was a heavy calendered 'art' paper with a
china clay loading mounted on board. They knew of no known additives
in the china clay. The china clay had a water based binding agent,
and the ink was similar to a Rotring / Isograph ink, but the
manufacturers would not tell them any more.

Of the samples I have experimented with, I have found the following:

1950s-70s scraperboard mounts react to produce a red colouration
when tested with Phloroglucinol and HCl, indicating the possible
presence of lignin.

Board produced from the 1980s onwards does not produce the same
result although the fibre appears similar under a microscope. This
would seem to suggest that the makeup of the mount substrate changed
from mechanical wood pulp to chemical wood pulp sometime in the
1980s. 1950s-70s board had a pH range between 2 and 4 while board
made from the 1980s onwards appeared to be stable around pH 6, I
would suggest that a buffer was also included during this change.

Examination under a UV light indicated the presence of an optical
whitener in the loading for all samples.

The binding agent appears to vary from manufacturer to manufacturer,
but seems to be either starch, Polyvinyl-alcohol(PVOh), or a mixture
of both. As potassium iodide will give a blue complex for both
starch and PVOh, it is possible to ascertain if starch is the sole
binding agent, but not if PVOh is the sole binding agent by
performing these three steps:

    1.  Perform Potassium Iodide test

    2.  Soak material in alpha-amylase preparation

    3.  Perform Potassium Iodide test

If a blue complex is found for step 1 and not step 3, the binding
agent is starch. If a blue complex is found for both steps 1 and 3
then the binding agent is either PVOh or a PVOh/starch mix.

I have no information about the ink surfacing of black scraperboard
as I have only had white samples to experiment with, mostly procured
from unused edges of my father's artwork, so any information on the
ink composition I would appreciate for my notes.

Jonathan Farley

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                  Conservation DistList Instance 10:56
                Distributed: Tuesday, December 10, 1996
                       Message Id: cdl-10-56-002
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