[NHCOLL-L:4107] Re: Removing Shellac (?) From Bone

Thu Dec 4 14:50:22 EST 2008

It could also be animal glue.  A test with boiling/hot water on a swab
will be able to tell if it is. It seems the most likely one to me to be
coming off in the manor described.  

Chris Del Re

Senior Conservator

MPM

From: owner-nhcoll-l at lists.yale.edu
[mailto:owner-nhcoll-l at lists.yale.edu] On Behalf Of Victoria Book
Sent: Thursday, December 04, 2008 12:48 PM
To: kwellspring at amherst.edu
Cc: NHCOLL-L at lists.yale.edu
Subject: [NHCOLL-L:4106] Re: Removing Shellac (?) From Bone

Hi Kate,

I feel the need to preface any comments regarding this type of invasive
remediation with an admonishment of extreme caution and a recommendation
to employ a conservator.  There are a number of pitfalls, from your own
health and safety to potential damage to the skull, in undertaking a
treatment of this kind.

That said, we all know that many collections do not have the resources
to employ a conservator, and such activities must sometimes be
undertaken by collection managers and other skilled staff.  When doing
so, please follow the procedures used by conservators, such as noting
the condition before, during and after treatment (including
photography), testing methods on a less visible or important areas
before proceeding, and documenting exactly what methods and materials
were used for the treatment.  Also use appropriate personal protective
equipment.

Shellacs, polyurethanes, cellulose nitrates and many other traditional
varnishes age extremely poorly.  They darken over time, are often acidic
(bad for a number of reasons), and shrink, pulling away from the
substrate (skull in this case) causing damage to the surface.  Also,
coating are often soluble early on, but cross-link and become fairly
intractable over time.  You do want such a coating removed (or not
applied in the first place) if it can be removed safely.  There are
three typical ways to remove a coating: mechanically, using solvents,
and ablation.  A combination of methods may be appropriate for an
effective treatment project.  

Mechanical removal using blades and sharp tools is the "cleanest", but
is a physical risk.  You are pretty much guaranteed to damage the
surface even more if you use only mechanical methods.  Mechanical
methods also tend to leave residues in bone interstices and canaliculi
which may continue to cause damage, or at least will look very odd as it
continues to age.   Solvents can be very effective in solubilizing
and/or softening the coating, but it may also cause components of the
coating to penetrate deeper into the bone. If you can identify the
coating you could narrow down your solvent choices, but probably won't
make a huge difference about which technique to use.  Ablation is a
relatively new method that shows a lot of potential, but probably has
associated risks, and is usually outside of most people's means.

(a) FTIR of a small sample (a "self-sampled" flake, or extracted from
the surface with a sharp scalpel) would be the easiest way to identify
the coating, but may not actually help much since you'll need to test
solvents anyway.  UV light will make a coating fluoresce: shellac
fluoresces orange, polyurethane and PVA look blueish-white, and
cellulose nitrate looks greenish-yellow, but fluorescence can be
difficult to interpret if you aren't used to it and the technique is not
really diagnostic.

(b) I would recommend using solvents to dissolve/soften/gel the coating,
then use cotton wipes, stiff brushes, and wooden tools (broken or
whittled ends of a swab are great for this) to gently scrape off the
softened coating, and wood and/or steel tools to flick off residual
flakes.  Mixtures of ethanol and acetone are a good place to start (try
applying solvents as a poultice using cotton pads (Webril wipes or
similar) or a gelling agent to keep the solvent in contact with the
coating for several minutes), but if those solvents aren't strong enough
you can try toluene and xylene, and even 1-methyl-2-pyrrolidone (pretty
nasty stuff health-wise).  If you use a poultice or gel, make sure to
clear (remove all residues) with a free-solvent.

Do NOT soak the specimen in a solvent bath.  You can use a UV light to
examine the skull during treatment to see where the coating still is; it
often remains in cracks.  You want to get off the entire coating and not
leave residues (of the coating or of the chemical mixtures) anywhere. 

Also be aware that chemicals are a risk to the bone, and may stain,
bleach or leach bone, and may also affect future analysis.  You (and the
curator) will have to determine whether the benefits outweigh the risks.
Proceed at your own risk...

Just my thoughts; does anyone else want to chime in?

Good luck,

Victoria

Victoria Book
Conservator
Sam Noble Oklahoma Museum of Natural History
University of Oklahoma
2401 Chautauqua Avenue
Norman, Oklahoma   73072-7029
phone 405.325.5163
fax 405.325.7699
www.snomnh.ou.edu
<https://exchange.ou.edu/owa/redir.aspx?C=2fbf6a889ace4e95a804c3871290ea
0d&URL=http%3a%2f%2fwww.snomnh.ou.edu> 

On Dec 3, 2008, at 3:27 PM, kwellspring wrote:

Greetings,

There's a lion skull in our osteological collection that is coated with
a splotchy yellow glaze that I suspect may be shellac (although it could
also be a polyurethane or other varnish). It is starting to flake off
and I'm concerned that it will be carrying away pieces of bone as it
continues to degrade. If it would not be of further harm to the
specimen, I would like to remove the glaze.

Does anyone know of a way to 
A.) Identify the glaze?
B.) Use a solvent or other agent to remove it?

Are there any publications that I can consult?

Thank you in advance.

Kate Wellspring

_____________________________________
Kate Wellspring
Collections Manager
Amherst College Museum of Natural History
Amherst, MA 01002
(413) 542-2165
www.amherst.edu/museumofnaturalhistory

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://mailman.yale.edu/mailman/private/nhcoll-l/attachments/20081204/ec062e20/attachment.html 