[NHCOLL-L:4106] Re: Removing Shellac (?) From Bone

[Victoria Book]

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



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

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