Repairing parchment with collagen

Jack Thompson has, unawares, fallen foul of the all too common,
woolly and inaccurate use of the term "collagen" here in the UK.
Christine and others insist on using this word when in fact they
mean sausage and other casings made from reconstituted collagen.
This will help to explain Christine's reference to "a dubious
flirtation with gelatine" (sic). It may be useful to give a more
precise definition of the term.

The pure collagen molecule is a stable scleroprotein; it is a triple
helix consisting of strings of amino acids with peptide linkages.
It is the most abundant protein in all animals (mammals, birds,
fish) and is identified by the existence of hydroxyproline as one of
the amino acids, which only exists in collagen. Each species of
animal produces slightly different collagen, the difference being in
the proportion of glycine, proline and hydroxyproline and the other,
less common, amino acids in the molecule.

As Jack rightly points out, isinglass is a form of collagen--fish
collagen or ictheogen (I think I may have spelt this wrong); it is
the swim bladder of certain fishes and 'isinglass' is simply its
(Scandinavian?) popular term, used especially by brewers who use the
material ground up to clarify beer etc. Christine seems to be using
the term to mean ground up fish collagen made into an adhesive.

In book and archive terms, collagen is most frequently found as
parchment or vellum. During its transformation during production of
parchment, skin undergoes substantial changes, the principle one in
this context being the fact that several other skin components are
dissolved and scraped away to leave just three: collagen 85.4%,
water 13% and residual lime (calcium carbonate) 1.6% is a typical
final composition. The collagen fibres are not reduced in size and
their naturally grown network is retained. This network simply
loses the non-collagenous matter (fats, proteoglycans, plasma
proteins) that exists between the collagen fibres in raw skin.

How does this compare with reconstituted collagen used commercially
in meat wrapping?

Dominic Wall of Suffolk Record Office recently approached some
casings manufacturers and asked for detail of their material. He
sent me this information for consideration and comment. The
material produced by Naturin Ltd for example, cited the following as
uncoloured material:

    collagen                   65%
    water                      13%
    glycerol and sorbitol      20%
    ash                         2%

The pH value was cited as a parameter at 4.0 - 7.0.

Devro Ltd quoted

    collagen                   70.5%
    glycerol                   20.3%
    sorbitol                    5.3%
    chloride (as HCl)           2.7%
    fat                         1.0

The process used to get the collagen dough is basically animal
products (cattle bits) reduced in HCl--hydrochloric acid--(or
similar depending on the producer). One process description (quoted
in the past by Maria Woods in her paper discussing the use of
casings for parchment treatment several years ago and later by
myself in the Journal of the Society of Archivists Vol 16 no. 1995)
says the reduced material is ground up and added to: "...acid which
causes the cells to swell and burst. The mix is spun extruded
through a machine into a bath of aluminium sulphate and buffered to
pH7 and very lightly tanned, then bathed in a mixture of alum,
ammonium sulphate and citric acid, after which it is washed and
made supple with glycerol, air dried for 10 minutes at 70 deg C and
cured for about 24 hours at 80 deg C."  Phew!!

I don't know how this particular recipe compares with those
currently in use. Some readers may be reassured that the remaining
hydrochloric acid in the Devro material (and presumably its
equivalent as ash in Naturin's) is only 2.7%. They should not be
reassured. To put it in perspective, when we as conservators
deacidify/buffer with magnesium or calcium bicarbonate we are
seeking to deposit magnesium or calcium carbonate to actively
protect documents etc for a long time. The residue left after
treatment can be considered very good if we manage to impart 0.5%
carbonate residue.

The manufacturers quoted 'shelf life' periods of a year. One
stated: "The product can be stored at room temperature under a
relative humidity of <65% for at least 12 months in the closed
original packaging." Presumably what happens if you remove the
material from its packaging is just what I have found when looking
at casings stored in my (controlled) lab. A steady loss of
flexibility leading to embrittlement. Hardly surprising when you
bear in mind that the collagen fibres have been reduced in length to
a fraction of their original, natural size in the acid preparation
process. The glycerol and sorbitol will be there presumably to
ensure the casings keep a little of their flexibility for the time
between being wrapped around sausage meat and being eaten--I
suggest not a very long period!

Can I make my message any clearer? This material is not intended
for use in conservation.

If we contrast this with goldbeaters' skin, which has been around as
a repair material for centuries and has been proven as stable, I do
not understand why some people still wish to pursue casings as a
suitable alternative. The only consideration I have been able to
find is that it can be got free from some of the producers. When
you consider that new parchment used for infilling costs
considerably more per square inch than goldbeaters' skin but the
casings users do not balk at buying and using large quantities of
it, you may well ask why the cost of goldbeaters' skin is considered
such a difficulty.

I know that Christine has in the past stated that she found that the
application of goldbeaters' caused some slight curling of the
parchment, suggestive of undue shrinkage of the goldbeaters' patch
during drying. In ten years of use I have only observed this effect
when too much gelatin adhesive was applied and when the goldbeaters'
skin had been over-stretched during its preparation--both effects
found in material treated when I was still training and had not
mastered its careful use.

I agree with Jack entirely about the use of starch for treating
parchment. The incompatibility of starch; its lack of hydrophilia
and the effect that this has on a hydrophilic material such as
parchment; the long term chemical retrograding of the amylose content
of starch into a non-soluble cross-linked solid; all these factors
combined with the shrinkage of the parchment caused by the liquid
water necessary to make starch paste usable and to subsequently
reverse it, all mitigate against starch being used to treat
parchment.

As if this wasn't enough, gelatin adhesive used for the application
of goldbeaters' skin is simply collagen in solution with water.
Unlike the vast majority of other colloids (including starch paste)
gelatin does not display syneresis during drying: i.e. the
consolidation of the solute and subsequent exclusion of liquid
water. To put it another way, starch and other colloids will
separate out during the drying process, allowing liquid water to
penetrate the parchment. Gelatin can only lose water as a vapour. As
has been amply demonstrated elsewhere, the only genuinely safe way
of humidifying parchment is by the use of water vapour (I do not
mean cold steam here, which is liquid water in droplets). Parchment
will be adversely affected by shrinkage on drying in air if liquid
water is applied to it, but parchment is able to take up and lose
water as a vapour without damage. Gelatin is, then, not only the
same material as the parchment being treated (especially if it is
derived from parchment scraps) but it is also not damaging if used
properly as an adhesive.

For those interested I can strongly recommend Betty Haines' paper
The Physical and Chemical Characteristics of Parchment, Casings,
Goldbeater's Skin and Gelatin, published by the Society of
Archivists in 1994 in the proceedings of its annual meeting for 1993
entitled Conservation for the Future, Dorset AIM. Perhaps it is
time for a re-print of the paper for wider circulation?

Chris Woods
Dorset Record Office

                                  ***
                  Conservation DistList Instance 10:89
                 Distributed: Thursday, April 17, 1997
                       Message Id: cdl-10-89-005
                                  ***