18th century painting preservation technique

I recently came across the method to preserve paintings outlined
below.  This is not my discipline, and I wondered how common or
unusual it might have been in the last quarter of the 18th century.

The dry painted surface is first brushed carefully, either with
water or a "secret" varnish, and glass exactly the size of the
picture is applied directly to the surface.  This is allowed to dry,
and the reverse of the painting is then coated with lead white.  A
piece of lead, slightly larger than the picture, is painted with
copal varnish.  When dry, the varnished side is applied to the
reverse of the painting and the edges of the lead are folded up over
the glass.  There is a combination of suet and wax that figures into
the description too, but I'm not 100% clear where. Context suggests
it may have been used for miniatures, although the author is
recommending it for large pieces to be sent abroad.

The memoire gives very specific reasons for each step, which I would
gladly share with anyone interested.

Any enlightenment would be greatly appreciated,

Sarah Lowengard
Paris, France

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                  Conservation DistList Instance 10:77
                   Distributed: Monday, March 3, 1997
                       Message Id: cdl-10-77-002
                                  ***