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Subject: Contaminated shellac

Contaminated shellac

From: Jim Moss <clkmkr>
Date: Friday, March 13, 1998
A colleague of mine who does not have network access has asked for
information and guidance regarding contamination that he has found
in shellac. Please reply to me and I will forward the information to
him. His information and request is as follows:

The problem: Contamination has been found that resists centrifuge
process for removal. He suspects that the contamination is a modern
additive used during the refining process. What is it and how to
remove it? Contamination is insoluble in common aromatic and
aliphatic hydrocarbons. He suspects that this contamination causes a
mechanical breakdown of permeability by penetrating the dried film
thickness and acting as a wicking thus contributing to pitting
corrosion of metal substrates.

Contamination characteristics: Acicular crystal formation, 10-12
microns wide, 40-60 microns long. Can only be seen with crossed
polarizers, Brownian movement very active in .1 ml drop of stock
solution on microscope slide until evaporated. Crystals orientate
radially to meniscus of drop at periphery of dried film.

Solution Preparation: Dissolve to saturated solution, lemon shellac
by magnetic stirring in absolute grain alcohol. Takes approx. 48
hrs. without heat. Centrifuge in 50 ml volume at 5 g's for 30 min.
All gross particulate separates into 3 distinct layers. The
remaining particulate is acicular crystal. Centrifuged over an
extended period of time at 5 g's with no effective separation.
Particulate can be separated for examination with alcohol wash by
continuously diluting a small volume of stock solution and separate
crystals by centrifuge for examination free from shellac. Clean
crystals in a .1 ml drop of alcohol are extremely active. Eventually
upon evaporation, the single crystals coalesce to groups of white
clusters. Prior to evaporation, crystals are transparent and cannot
be seen without polarization.  Reducing saturated stock
concentration to 30% by volume does not apparently improve removal
by centrifuge technique. Fritted glass filter under vacuum clogs
instantly and is ineffective and as the solvent for the crystals has
not been found, the filter cannot be cleaned.

Need: references on studies performed, sources of books and
technical data on shellac as well as information regarding refining
processes used in Europe and UK during early 19th century as well as
what changes have occurred in the modern day refining process.
Period of interest is 1800 to 1900.

Application: coating of early American scientific instruments.

Jim Moss

                  Conservation DistList Instance 11:77
                  Distributed: Monday, March 16, 1998
                       Message Id: cdl-11-77-012
Received on Friday, 13 March, 1998

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