Thank you Karin for such a thorough
and thoughtful reply, and Jennifer for pointing out the important questions.
When the question about interleaving tissue came up I knew it would
generate some very interesting discussions and would also be very interesting
to know what is the current thinking. Having been involved with paper
and textile research these many years, and learning about risk assessment
approach, I have my own perspective on the topic.
I agree with Jennifer, the most important
question is what is the expected function of the interleaving tissue. If
we go back to Diana's original question:
1. Is it necessary replace interleaving
tissue on a regular basis?
I can't recall any systematic study
that showed the benefit of replacing tissue on a fixed time interval, or
evidence of damage from tissue that is not replaced regularly. I
would love to hear about incidents contrary to my recollection (which is
deteriorating quickly with age). In my opinion, the need to replace
tissue depends on the cumulative volatile contaminants in the environment,
both externally or internally(from off-gasing or degradation products),
the thickness and chemical composition of the tissue, temperature and relative
humidity etc.
2. What is the expected function of
the interleaving tissue?
In my observation and experience, interleaving
tissues are used more as a physical barrier from dust and dirt, and a sacrificial
barrier to absorb some acidic or oxidizing contaminants, or to prevent
transfer of colourants. As a chemical barrier (ie. absorb or neutralize
acidic contaminants) it is relatively ineffective because of the thinness
of these tissues. The tissues (if they are the kind that we buy)
are so thin, and if they are not buffered, they are not an effective barrier
of acids. My thinking is always that a little bit of barrier is better
than no barrier, unless resources is an issue, ie. money to buy the tissues,
manpower to change it etc.
3. When should tissues be replaced?
Others on the list have already commented
on this: if the tissue is in good condition - not yellowed (from oxidation),
no colourant transfer, not acidic or not physically torn, it does not need
to be replaced. A quick check of the pH of the tissue is a very important.
As the pH tells you about the environment. An acidic tissue
is a good indication of how acidic the immediate environment is and also
that the tissue itself will be brittle and will no longer serve it's function
- so it needs to be replaced.
4. What kind of tissue or interleaving
materials should be use? What about alkaline tissues on protein?
The question of using alkaline tissues
on textiles is a contentious one. I was asked once to provide references
of why alkaline tissues should not be used on silk, wool or textile with
colourants. I couldn't find any other than the fact that certain
natural colourants changes colour with pH - a well established fact, and
that protein fibres silk, wool or collagens are more stable in a slightly
acidic environment (pH 5-6). In highly alkaline solutions (eg. lime
or sodium hydroxide at pH 12) proteins (and many other things) swell, break
down and turn yellow . I couldn't come up with conclusive evidence
that protein fibres wrapped in alkaline tissues causes any problems. Again,
I would love to hear about your experience on this. But it is something
that I was told and I just believed, until recently.
What made me question this is the following:
- First of all, the compounds in alkaline
tissues are typically calcium carbonate. It has a very low water
solubility. Unlike acidic degradation products which are small volatile
molecules that migrates, calcium carbonate is a solid particle, they do
not migrate, and therefore cannot change the environment just by being
in the vacinity. In Volume 79, N. 11 of the Tappi journal, scientists
from the Pulp and Paper Institute of Canada published the results of their
a new method of "dry deacidification" of paper, where they place
a sheet of acidic paper between sheets of paper containing calcium carbonate,
humidifying these sheets and then subjecting them to high pressure. With
sufficient moisture (to dissolve the calcium carbonate) and extremely high
pressure to ensure intimate contact, they reported neutralization of the
acids in paper.
To put that work in the perspective
of textile storage, in order for the immediate environment of the textile
to change to alkaline (from the use of alkaline tissues) there need to
be near 100%RH and intimate contact with the substrate. One possible
risk would be if there is a flood or somehow the tissue got wet and was
allowed to stayed on the textile, if there are alkaline sensitive colourants,
perhaps the colour may be affected. The benefit of using buffered
tissues is that the tissues themselves will last longer especially when
it is in an acidic environment. Are they better barriers of acidic
volatiles? Possibly.
- The second thing that made me question
the 'risk' of buffer tissue is, a lot of these protein fibres gone through
alkaline processing during their manufacturing. For example, silk
is often boiled in alkaline soap solutions for degumming. Parchment
is prepared by soaking the hide in lime solution to remove the hair and
epidermis layer, rinsed to remove the alkali, later on calcium carbonate
is rub on the surface to improve the writing property (keep inks from feathering),
and the calcium carbonate remains in the parchment. We have seen
parchments that last hundreds of years in an alkaline environment.
- Lastly, in the early 1990's the CCI
evaluated 3 mass deacidification systems. One of the study we subjected
thousands of pieces of test materials to mass deacidification treatments
- all of them leave behind Mg-carbonate as a buffer, and it has a higher
pH than Ca-carbonate. Some of these materials include photographs
with gelatin and albumin coatings, leather book covers, blue prints and
water colours - materials that we expected to change colour because they
were "not supposed' to be subjected to alkaline treatment. What
we found was that there were no noticeable damage (colour change, physical
damage) from these protein (collagen) materials. The only colour
change of the watercolours was the gamboge, but all the other colourants
that we expect to be alkaline sensitive did not change colour. We
attributed this to the non aqueous treatment - somewhat analogous to the
buffer tissue situation. Now 15 years later, we noticed that some
of the untreated controls have changed colour while the treated samples
retain their original colour.
I don't have proof that alkaline buffer
tissues pose no risk to protein fibres, but it does help me be more flexible
with my choices.
One thing I noticed, why isn't washed
unbleached cotton sheeting suggested as an alternative instead of tissues
or Reemay or Tyvec? It may be too obvious, since it is used for so many
things in textile conservation. Is it also used as an interleave?
It didn't come up as an alternative in this discussion, I just wondered
why not. It seems quite ideal, other than that you cannot see what
is wrapped inside or underneath and it adds abit of bulk. It is durable,
it doesn't have the problem of buffer, it 'breathes' and it doesn't tear
easily.
From a risk assessment perspective,
I would ask, if I have limited resources, should I put my money on replacing
the unbuffered tissues every X number of years irregardless of their condition,
or are there areas of higher risk that should be addressed? If by
choosing a more durable interleaving material, it will allow resources
to meet other needs, that seems to be an important consideration.
But the one thing I learned, is that
once a concept is ingrained it is very difficult to change, even though
the evidence points the other way. I still find myself reaching
over to the unbuffer tissues to wrap everything - just in case.
Just a few thoughts.
Season
Season Tse
Senior Conservation Scientist/Chemist
Conservation Research
Canadian Conservation Institute
Canadian Heritage
1030 Innes Rd.
Ottawa, ON
Canada
K1A 0M5
Tel: (613) 998-3721 x-187
Fax:(613) 998-4721
email: season_tse@xxxxxxxxx
Websites: www.cci-icc.gc.ca & www.preservation.gc.ca
Jennifer Barnett <jcbarnett@xxxxxxx> Sent by: Textile Conservators <TEXCONS@xxxxxxxxxxxxxxxxxx>
28/05/2007 07:29 AM
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Textile Conservators <TEXCONS@xxxxxxxxxxxxxxxxxx>
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Subject
Re: Interleaving tissue
Dear all,
I am glad Karin has pointed out the practical aspects
of interleaving tissue that should be considered in the storage of textile
collections. It is good to go back to basics regularly.
It seesm to me that the use of tissue seems to have become
'traditional' with the original function having been lost in the mists
or time, or, should I say, in countless layers of creased, reused tissue.
The most important question here is, in my opinion, what
are you trying to achieve with the use of interleaving tissue in
a given collection or part thereof in the first place?
Exploring this question rationally could reveal other
possibilities and solutions. For example, the use of interleaving tissue
may turn out to be unnecessary in many cases. That certainly would free
time and resources for more urgent matters.
Sounds like a nice student research project...
Best wishes,
Jennifer Barnett
Regina Textilia
not only historic textiles conservation, research,
training and consultancy but also translations (Dutch-English)
Route de Dancé
FR - 42260 Bully
On 28 May 2007, at 11:50, Karin von Lerber, Prevart GmbH
wrote:
Only "Unbuffered" acid free tissue should be
used with textiles.
Buffered tissue with an alkaline reserve is not an appropriate
material
for the archival storage of textiles.
I think, this generalization is not completely correct.
For cellulosic textiles (cotton and linen etc.) buffered
paper is just
perfect, however silk and wool generally need even a slightly
acidic pH
as their most stable environment.
The problem really is, that textiles, as well as many
other kinds of
objects in mixed media collections (e.g. Historical Museums)
rarely come
as pure cellulosics or pure animal fibres.
I think this really points toward a field of research
desperately needed:
we do know about possible damage of using the "wrong"
pH with certain
groups of pure materials. But: how do these dangers compare?
Is it a
higher risk to store a linen fabric with silk embroidery
and metal
thread in neutral, in slightly alkaline (buffered) or
in slightly acidic
(nonbuffered, aged) paper?
Ideally a museum would have to stock three different types
of tissue and
- as posted at the beginning of this thread - change it
regularly.
However, experience in real life proves, that keeping
track of the type
of paper is impossible (they all look nearly the same)
and paper is
frequently "transferred" from one object to
an other one when working
with the collection. Therefore, having three different
types of paper
available and using them correctly is just not realistic,
nor is the
idea of constantly walking around with a pH-testing pen
(cost of
manpower!) The question asked above thus becomes even
more urgent: what
type of paper will pose the least risk to the bulk of
a mixed
collection? (Please note: I am not talking single objects,
but collections)
I think the research needed would have to go into the
kinetics of aging
fibres and the change in this kinetics by direct contact
with the
different types of paper under museum environment conditions,
and under
not quite as ideal climate conditions (church, basement,
attic). And
there will have to be research into how long it will take
under what
climate condition for the various types of paper
in contact with
objects to change significantly (i.e. how long will the
paper/ cardboard
box really provide the storage condition postulated?);
this being a
ressource question (manpower, cost of material renewal).
With such research data, a calculated risk assessment
would become
possible, and each museum / institution could adapt this
risk assessment
to their specific collection composition. Such a risk
assessment will
have to include a comparison of the multiple factors
of risks and
benefits of the cellulosic storage materials to the risks
and benefits
of other (manmade) materials.
All of this hopefully enabeling us to preserve our collections
on a long
run with more and more restricted ressources.