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Nicole inquires about cmc of Orvus WA paste. I
cannot answer this directly, but I think the reason, why you find different cmc
values in the literature (and different "recipies" on how much detergent should
be added to the wash bath) is the following: Instead of rising the price, the
producing industrie is mixing more and more water into the "concentrated"
detergent. Therefore we - in Switzerland - measure the cmc for every batch of
detergent we buy. This is very cumbersome, but when you split a larger batch
among colleagues, only one of them has to run the measurement... We have
observed for Tinoventin JU (Alkylphenolethoxylate, a non-ionic detergent
produced by Ciba) that cmc has risen from 0.5 g/l to 2.2 g/l during
the last 10 years!
My experience has shown, that a lot of colleagues
are not familiar with the method of how to measure the cmc. It is based on the
fact that for a drop in its equilibrium on the tip of your burette, the surface
tension multiplied with the circumference of the droplet is equal to the mass of
the droplet multiplied with its weight. Translated into practical terms this
means: you need a burette and pure water (of the kind you are going to use.
Caution: the cmc is different for each type of water (softness)!) and different
dilutions of your detergent in this water. You start with water. Make the fluid flow *very* slowly in your burette, so it forms
big drops in equilibrium before the drop falls. Count 50 drops and weigh them.
Then calculate:
Surface Tension = 72.8 x (M'/M). M stands for
weight of H20; M' for weight of H20+detergent; 72.8 dynes/cm3 = surface
tension of pure water.
Repete this for all the dilutions and plot the
surface tensions in a graph with surface tension on its vertical and with the
concentration of your detergent solution on its horizontal axis. You will end
with a curve falling from the top left and smoothing out to the right. The cmc
is the point at which the curve becomes flat (because adding more detergent
won't lower the surface tension any further).
Now to the second part of the question: should the
detergent be chosen at or above cmc. I think, that depends on the soil you want
to remove and on the detergent-combination you intend to use. E.g. if you are
going to add an emulgator, you will need less detergent, because the emulgator
will do some of the "carying" work. But if you are trying to remove oily soil,
or if you can't use CMC, you need enough detergent so the soil can be wrapped.
So: as in many domains of textile conservation: no
recipies... It might be worth consulting Agnes Timar's excellent literature
rewiev : "Wet cleaning historical textiles: surfactants and ogher wash bath
additives" in: Reviews in Conservation Number 1 2000 (IIC),
p.46-64.
I hope this helps and apologize for my English.
It's not easy to write things like this in a foreign language, and maybe Dinah
Eastop, Mary Ballard or someone else knows where well-written procedures can be
found in English?
Good luck!
Karin von Lerber
______________________________ Karin von Lerber
Prevart GmbH Konzepte für die Kulturgütererhaltung Oberseenerstrasse 93 CH-8405 Winterthur Telefon + 41 (0)52 233 12 54 Fax + 41 (0)52 233 12 57 e-mail: karin.vonlerber@xxxxxxxxxx www.prevart.ch
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