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bronze, melting temps, pit firing: loooooooonnnnnnggggg part 1

updated sat 27 apr 02


Craig Clark on fri 26 apr 02

Karen, this is part one of what will be a 2 part response to your
question. First, I'm assuming that you will be using a silicon bronze for
your casting material.
The fusion (melting) point of Bronze is considerably higher than the
temp that you would normally achieve in a low fire pit. Considering the case
of the pit not going much above 1400 to 1500 degrees F.
If the pit temperatures that you are able to achieve (I don't know much
about pit firing) go into the bisque temp and/or mid fire ranges with temps
exceeding 2000 degress F then you will indeed have a molten blob on your
hands after the firing.
As to the question regarding the patination of Bronze, think of this
process as being similar to what you see when either Iron or mild steel
rusts. That gorgeous orange tint (I think it's gorgeous anyway) that forms
is a patina. It is the result of the the carbon in the iron in the metal
reacting with the oxygen and moisture in the atmosphere.
The "color" that is seen is the result of a change in the physical
characteristics of the metal on the surface (the rusted part.) This physical
change, to put it simply, results in a different portion of the visible
light spectrum being reflected form the surface, therby "imparting" a change
of color. If you would like a more scientific or detailed explanation of
color theory and the physical changes that occur then perhaps someone on the
list that knows something about physical chemistry can help you out. This is
as far as I can really go.
When you patina bronze, you are applying a certain chemical or substance
to the surface to bring about a reaction with the surface of the bronze that
changes said surface. This is the "rusting" of the bronze. There are a wide
array of materials and techniques available to bring about an equally wide
array of results. There is an exceptional book on bronzes and the patination
therof. I don't remember the title but will probably be able to find it
Keep in mind that the colors that you see are the result of a chemical
reaction. The structure of the surface of the bronze is actually changed.
Sometimes, if hast is required, I've seen folks heat the bronze the were
using with a torch to speed up the process. This was done with varying
degrees of success and I believe resulted in the immediate release of some
particularly noxious fumes. In the cases that I saw ammonia and liver
sulphate were being used.
As far as an exisitng patina "burning off" in a low fire pit, I don't
think, though I may be mistaken, that it would unless the temp of the pit
began to approach the fusiion point of the bronze. Sorry, I don't remember
the temp, it's been better than ten years. I'll look these numbers up and
supply them in part two.
One thing that I can state emphatically is that rather than spending
your time, money and energy on a number of bronze casting that may or may
not work with the desired "smoking" effects that you are looking for in a
post patina pit firing I would suggest that you make several bronze test
strips. Just like we do when testing glazes. Try them out and see what does
or does not work.
An alternative would be to try the various fumimng techiniques of
patination that in themselves can produce the wispy characteristics of smoke
in the color that are also quite nice.
Hope this helps. I'll find that book, ask my foundry pals a few
questions, and get back to you with some more specific info.
Craig Dunn Clark
619 East 11 1/2 st
Houston, Texas 77008

----- Original Message -----
From: "karen lovenguth"
Sent: Friday, April 26, 2002 11:06 AM
Subject: bronze, melting temps, pit firing

> Hi all,
> I have been following a bit the discussion
> about patinas and bronze. My claywork is pit fired and
> finally not very utilitarian. That is just its nature.
> I have had some thoughts though about casting a few of
> my pieces in bronze to be able to use as a pot for a
> plant. Many of my pieces are inspired from plant
> forms. I would love to be able to plant a certain
> plant in a certain pot but as I said low temp pit
> firing work doesnīt like to get wet and finally will
> not wear well to an outside climate. I was wondering
> if I were to give a patina to a bronze piece,then
> could I put it in a pit and get the effects of the
> fire and smoke,oxidation and reduction? Or will it
> just melt? Has anyone had any experience with that? My
> husband has lots of bronze sculptures here but I donīt
> think it would be a good idea to try out my idea on
> them!! Hope someone can help.Karen
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