Gilding

Jim Moss <clkmkr [at] tiac__net> writes

>I would like to inquire of the Cons DistList members who work on the
>preservation of buildings the same question: how long would the
>gilding on a wooden object be expected to realistically last (an
>example might be on a dome of a building) in an oceanside
>environment: extremes of temperature, very high winds, lots of
>moisture, driving rains, strong sun, most likely lots of airborne
>particulate and salts? Are there special techniques, materials, or
>sealants that need to be used to increase the longevity? Does
>gilding come in various thicknesses?  From conversations with some
>other conservators, the use of protective coatings was discouraged.

Unfortunately to my knowledge, there is no standard specification
for gilding. However, after 35 years conservation gilding experience
I would estimate that properly applied, an exterior gilded surface
should retain its original appearance for 25 to 30 years. Improperly
applied, it will disappear in a fraction of that time. The most
probable answer as to why some exterior gilded objects last for over
25 years while others begin to fail within 6 years, is almost
certainly that the surface had been inadequately prepared and the
priming medium used was unsuitable in its ability to bond with the
substrate. We cannot emphasize strongly enough that the art of this
form of decoration is in the preparation.

If a gilding project is to achieve its maximum life expectancy,
there are several important factors that must be considered. First,
the substrate and its characteristics need to be fully understood.
Any existing coatings should be removed. Chemical removal is more
appropriate for older coatings containing lead as it keeps it wet in
the solution. Care should be taken in selecting the correct removal
agent as certain caustic removers (basically lye) can change the pH
of the wood. Though slower than traditional methylene chloride
removers we have had good results using water based "nontoxic"
removers based on n-methyl pyrrolidone. When removal is complete,
the surface should be neutralized.

After this step, the substrate should be coated with an appropriate
primer--I assume that apart from the gilding failure, the clock
hands presumably wood are otherwise in good condition without decay
or loss--The wood primer should be somewhat elastic in order to
expand and contract with the wood as temperature and humidity rise
and fall. The question of whether to use either an alkyd or acrylic
primer is debatable, and would require us to plunge into the deep
waters of polymer chemistry. Within oil based primers, oils with the
least degree of unsaturation have the potential for longer-term
flexibility than the more unsaturated oils such as linseed. We would
suggest a premium quality long oil alkyd primer. The proper primer
not only helps to protect the wood from rot but also helps to level
the surface and seal it so subsequent coatings will not be absorbed
into the surface. We would recommend 2 additional undercoating of
alkyd paint (yellow base for gilding) be applied. Sanding and
filling as necessary between coats. The high reflectance of gold
leaf is such that surface defects are accentuated by its use, and
therefore if gilding is to be effective the surface must be fit for
its reception.

For adhesive size, we would suggest avoiding the use of a quick
drying mordant on exterior work, as its high gum content tends to
make it brittle with age. Use a slower drying size. After coming to
tack, slow sizes have an open working time of 8 to 12 hours,
allowing more than sufficient time to complete the gilding. The open
time of the size will depend upon temperature and humidity, and
needs to be tested on-site to determine the proper interval between
application of the size and the laying of the gold leaf, and hear in
lies the art of the gilder perhaps.

Externally situated, Two layers of gold leaf should be applied to
help resist weathering. The alloy and weight of gold leaf should be
of extra heavy weight, with no less than 18 grams of gold per 1,000
sheets. The alloy should be 23.5 to 24-karat gold a higher gold
content makes for a more durable leaf with darker tones. Double gold
leaf is thicker than regular leaf but only by 10 to 20%. A
protective coat of weak parchment size may be applied to the
finished gilded work. Of course, the work should only take place
under the best possible weather conditions, or better yet in the
studio. Perfection in execution depends on excellence of material,
deft handling of the gold leaf and a perfect initial surface. There
is much more I could say, but would take a book. Regards,

Ian J Rogan
The LV.Greyes Partnership
Calgary, Alberta T2T 0M4
Canada
403-228-2728
Fax: 403-228-1416


                                  ***
                  Conservation DistList Instance 14:6
                   Distributed: Friday, July 21, 2000
                        Message Id: cdl-14-6-004
                                  ***