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Fire and fire gilding


#1

Hi,

Do any of you know at what temperature mercury amalgam gilding
(‘fire gilding’) will burn off silver?

I know the fire gilding process is carried out at about 350 deg. C,
but at what temperature does the gold burn off - or quickly diffuse
into - the silver? I guess that someone with experience trying to
repair a mercury gilded piece might know the answer, but the reason
for the question is to find out whether niello work or even
low-temperature enamelling could be carried out after mercury
gilding. It is all prompted by an examination of some Medieval silver
jewellery where the apparent order of work is slightly odd.

Thanks and greetings
Jack Ogden
http://www.jpm-international.com


#2

Hello Jack;

First off, if there were any residual mercury present, as might have
alloyed into the silver when it was gilded, it could change
everything, since it would form a eutectic alloy and alter the
temperature at which the gold an silver would begin to alloy, and I
suspect that would be well below the temperatures at which most
enamels melt, around 1500 F. In any case, alloying of the gold and
silver would certainly take place long before the gold “burned off”.
But in enameling, we’re talking about possibly repeated firings, and
many reactions that are expected at one temperature can occur at a
lower temperature if the exposure is for a longer period of time.
Niello, on the other hand, has a quite low melting temperature
relative to these temperatures, off hand, I’m guessing around 700 F,
maybe lower, depending on the alloy’s proportionate elements. Should
be able to add niello to a gilded article, as long as the niello
doesn’t get on the gilded area, where it would adhere freely and
removal might also go through the gold layer. On these articles you
refer to, examine them for signs that these combinations of materials
might also have been achieved by mechanical means, for instance, you
could guild an article, then add niello, and if enamels were needed,
they might be attached my means of bezels. Or, the gold might not be
gilded but rather, inlayed, in which case, the enameling could be
done without concern unless the article remained at those
temperatures for too long. Niello might be added if it were applied
to the article as the enamel cooled but before it was too cold for
the niello to flow, then all you would have done would have been to
hold the enamel at a plateau during it’s normal cool down period,
then allow it to proceed to cool down. Also, historically, niello
was often applied in the same manner as enamels, in powdered form.
One could fire the enamels, let cool, then re-fire to a lower
temperature for the niello inlay, as long as the enamel didn’t
undergo any thermal shock from abrupt temperature changes. If I
were to guess the order, assuming it didn’t involve any fancy
mechanical solutions, etc., it would be guild first, then enamel with
a fairly low temperature enamel, then do a second firing at lower
temperatures to fire on the niello. I’ll be watching this thread to
see what the more technically literate among us have to say.

David L. Huffman


#3

According to the description of a silver cup done with niello and
amalgam gilding from Theophilus On Divers Arts written in the 12th
century, the niello was applied before the amalgam. This makes sense,
because that dangerous old process was typically done slowly at much
lower temperatures than niello would fuse or melt at.

Ron Charlotte – Gainesville, FL
@Ron_Charlotte1 OR afn03234@afn.org


#4

According to the description of a silver cup done with niello and
amalgam gilding from Theophilus On Divers Arts written in the 12th
century, the niello was applied before the amalgam. This makes
sense, because that dangerous old process was typically done slowly
at much lower temperatures than niello would fuse or melt at.

Ron Charlotte – Gainesville, FL
@Ron_Charlotte1 OR afn03234@afn.org


#5

Hi Jack,

The key factor would be how thick a layer was put on. I imagine you
are thinking of the danger of heating flash plating, which would
dissipate instantly. But mercury gilding characteristically gave a
much heavier layer of plating than today’s heavy (not flash)
electroplating, and it was usually done several times to build it up
even more. Theoretically it wouldn’t burn off until at least 890C.
(according to Littledale, whose chemistry was sometimes a bit
shaky). You are wise to seek someone who has had experience with a
similar object. But I imagine there is a very wide range of plating
thickness from object to object, so what holds for one is not
necessarily true of your object. Your claim that “the apparent order
of work is slightly odd”—do tell us more!

Janet in Jerusalem


#6

Fire gilding would be done after niello or enamel work. If you were
to take a fire gilded piece up to the temps of soldering or fusing
glass or niello the gold would diffuse into the silver and depending
on the temperature and time you will either have a very green gold
surface or no visible gold left due to the gold having diffused into
the bulk silver below it.

Jim Binnion

James Binnion Metal Arts
Phone (360) 756-6550
Toll Free (877) 408 7287
Fax (360) 756-2160


@James_Binnion
Member of the Better Business Bureau


#7

Hi Jim,

   Fire gilding would be done after niello or enamel work. If you
were to take a fire gilded piece up to the temps of soldering or
fusing glass or niello the gold would diffuse into the silver and
depending on the temperature and time  you will either have a very
green gold surface or no visible gold left due to the gold having
diffused into the bulk silver below it. 

I would never recommend fire gilding without lab conditions.

there is info at ganoksin:

I did it a lot at one time as a youth (phew I’m still here), maybe
50 or 60 times, and got good at it.

It is very controllable, and the gold is thick. My experience was
that the gold would take very high temperatures without any absorbing
into the silver, though I agree you could play with very thin layers
and diffuse them into the silver to get greenish alloys. I would do
niello last of all in such a procedure, but I think you might get
away with enameling onto a fire gilt surface.

I spent years learning to emulate the effects of fire gilding to
avoid doing it, as I now consider is unnacceptable. Keum boo is very
effective for certain looks. Marne Ryan has approached it in her
fusing of 18k onto sterling. Flame fusion (metalizing) and vapor
deposition (can be done by anyone with access to a scanning electron
microscope) work well.

keum-boo info here:

Here is a quote from my 1986 paper on gold application alternatives
to fire gilding;

  In the past I have used an valued fire-gilding as a way of
  placing gold metal surfaces.  There are however other ways of
  obtaining similar effects and in the light of today's knowledge
  of the toxicity of mercury this is a totally unacceptable
  procedure except under stringent, appropriate chemical lab
  conditions.  It is worth mentioning Mark Stannitz who did
  research on mercury gilding at RIT with proper lab conditions
  and gave it up as he could only obtain 95% recovery of the
  mercury used and felt this was too much unaccounted for.  In
  1987 when delivering this paper to the SNAG conference I
  approached Walter Soellner whose account of mercury gilding is
  the Untracht book.  I wished to apologize as I intended to
  criticize his procedure (admittedly the best of any
  published-still unacceptable).  His response was "Good I don't
  do fire gilding any more, its too dangerous...." 

  Fire gilding involved the making of an amalgam, a mixture of
  gold and mercury.  This mixture is a complex one involving
  mechanical mixing, chemical bonding, alloying and dissolution. 
  The heart of the process is that mercury 'wets' the gold. 
  Wetting is what detergent does to solutions and greases.
  (Gmelin, pg 289).  Excess mercury was removed from the amalgam
  by pressure leaving about 40% of the remaining amalgam as gold
  (Gmelin pg 283).  The amalgam was placed on the work and
  evaporated with heat to leave the gold behind as a deposit to
  be burnished as it has a rough, porous texture after firing. 
  While all stages up to the evaporation of mercury may be done
  under water (even this is not described in the literature) to
  minimize exposure the last heating stage is the part that
  releases mercury in volume. 

  It is fair to say that all published accounts I have seen on
  mercury gilding are unsafe, some downright dangerous.  In some
  cases no safety warnings at all are given (Diebeners, pg20) or
  its down played in statements such as "For enameled pieces,
  mercury gilding is less dangerous than electro-plating with its
  strong current." (Seeler, pg 79). Many accounts of procedures
  described in contemporary sources are virtually unchanged from
  versions recorded in Roman times or before as early as 700 BC
  (Gmelin pg 38).  This is despite changes in knowledge of what
  mercury does to the body.  It's effects include emotional
  stress, severe trembling, loss of muscle control, slurred
  speech, personality changes, osre gums, loose teeth, blurred
  vision (Quin, Stock, Pg20), lung and kidney damage (Untracht pg
  667) and often permanent damage to the central and peripheral
  nervous system (Quin, Stock, pg 20).  Death can be a result of
  mercury poisoning (McCann pg 16). I have recently been informed
  that mercury has a half-life in the body of sixty days or so
  (Canada Safety Council pg 1333).  Low level exposure (no one
  seems quite sure what low level would mean in fire-gilding)
  will eventually leave the body to a great extent.  Repeated or
  high exposures build rapidly to really toxic levels.  When
  fire gilding mercury condenses on tools, vent hoods, walls etc
  and re-evaporates at room temperatures creating a slow and
  constant exposure.  The health effects were noticed in the
  past.  The Incas banned the extraction of mercury and Cellini
  wrote "Gilding should be left to those whose vocation it is to
  do it, as it is..... extremely damaging to the health.  The
  Master is satisfied to know the procedure and with the-Basta:
  (Cellini, pg 96). Goldsmiths are often restricted in their
  thinking by tradition and mercury gilding procedures are a case
  in point.  The descriptions are unchanged in hundreds, even
  thousands of years.  It is possible that with research a safe,
  laboratory procedure may be developed, possibly by replacing
  the mercury with a non-toxic material which is removed leaving
  a powdered or flake 24k gold on the host surface to be
  burnished down.  At the moment however there are alternate
  procedures described in this paper which can give similar
  results without danger from mercury.  It was partly a search
  for such alternate procedures to mercury that led to writing
  this paper. 

Some at ganoksin:

and an article on doublee at:
http://www.silversmithing.com/1doublee.htm

best
Charles

Charles Lewton-Brain/Brain Press
Box 1624, Ste M, Calgary, Alberta, T2P 2L7, Canada
Tel: 403-263-3955 Fax: 403-283-9053 Email: @Charles_Lewton-Brai1


#8

Thenk you, Charles L-B, for the fascinating and scary article about
fire guilding.

It made me think of another method of applying gold I just heard of.
I think it was posted on SNAGnet, though maybe it was Orchid-- about
a 24k gold “paint” that is fired on, essentially a “luster”, as it is
called in ceramics. I’ve used this process on porcelain, and we all
have seen/owned china or glass with gold applied this way. So, here’s
the question-- has anyone tried this stuff on silver? I feel sure I
filed the post, but cannot now find it :>(. Hope you don’t all think
I’m nuts, which is how I feel when I can’t remember stuff… Noel