In the 80's, the US Forest Products Laboratory in Madison, WI conducted a
thorough study of vapor barrier properties of a host of coating materials.
While they did not specifically test B-72, acrylic finishes were included.
Following are relevant factors for your consideration. These are from
memory, as I can not put my hands on the report right now.
1) For virtually all materials, the thicker the coat, the better it was as a
vapor barrier. Two coats were better than one, three were better than two,
etc.
2) Coatings that included pigments generally were better than the same
material without pigments. The best pigment was aluminum flake. In fact,
traditional types of coatings with this pigment were the best vapor
barriers.
3) No material tested was a perfect vapor barrier. All eventually allowed
moisture to penetrate through it. Based upon their protocol which monitored
the samples for a number of weeks (somewhere around 8 weeks, but I don't
recall exactly), the best were around 90%-98% effective, the worst actually
INCREASED the rate of moisture migration (this was linseed oil at
about -2%).
4) Acrylic resins were in the 2%-6% range for 3 coats, as I recall, so they
are rather poor vapor barriers. In the transparent (non-pigmented)
coatings, wax (melted/dipped) was the best, up in the 90% range. Of the
more traditional brush-applied coatings, shellac and polyurethane were the
best, in the 30%-40% range for 3 coats. Adding pigments would increase the
vapor barrier properties.
I would suggest you use melted paraffin (or microcrystalline) wax. You can
also use dissolved, brushed wax, but this requires multiple coats to build
thickness. If this is not feasible, I would use shellac with flake aluminum
pigment added. If this is visually objectionable, you can top coat with
anything else. Or if you want a coating only, I would suggest shellac, with
a minimum of 3 coats. You can get flakes (dissolve in alcohol) that are
called super blonde, which is a very light yellow, if the orange color is a
problem. Do not use white shellac, as it is bleached to remove the color,
and some residues remain, as well as the shellac molecule is damaged and I
don't know if it will perform the same as regular shellac.
Marc
American Conservation Consortium, Ltd.
4 Rockville Road
Broad Brook, CT 06016
www.conservator.com
860-386-6058
*Collections Preservation Consultation
*Conservation Assessments & Surveys
*Environmental Monitoring & Low-Tech Control
*Moisture Management Solutions
*Collections in Historic Structures
*Collections Care Grant Preparation
*Conservation Treatment of:
Furniture
Painted Wood
Horse-Drawn Vehicles
Architectural Interiors
Marc A. Williams, President
MS in Art Conservation, Winterthur Museum Program
Former Chief Wooden Object Conservator, Smithsonian Institution
Fellow, American Institute for Conservation (AIC)
----- Original Message -----
From: "Dan Bartlett" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Wednesday, September 19, 2007 8:56 AM
Subject: -- Spam --[MUSEUM-L] B72 as Vapor Barrier
> Hey all:
>
> Will B-72 act as a vapor barrier? I need to drive some screws through the
> laminate inside a sealed exhibit case. I'd like to seal the recesses with
> 2-3 coats of B-72.
>
> Thanks,
>
> Dan
>
> --
> Dan Bartlett, BFA, MA
> Curator of Exhibits
> Midway Village & Museum Center
> Rockford, IL
>
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