Reid Harvey on sun 14 jan 01
Fellow Clayartists,
Yesterday I had the opportunity to visit the workshop of Mr. Sukanta Banik, whose brass casting, lost wax workshop is the last of an 800 year long tradition here in Bangladesh. Mass produced items are encroaching on Sukanta's market, and he needs help in improving his molds, in order to see that there be any chance of the tradition's continuity. (Sorry I have no pictures to share, since my digital camera is having mysterious problems.)
Sukanta has four wax sculptors who are the only one's left in the country who can fashion the bees wax into the sculptures/ models, used for the functional and religious pieces made in the workshop. But one serious problem is that the larger molds frequently break during casting. This means that long hours of work can be made fruitless. This problem with the molds is largely due to their being a bit thin and fragile, since they must be light enough to move from the kiln at the time of casting.
I believe that one related problem is that water content of the mold material used (50% clay, 40% sand and 10% jute fiber) is much to wet at about 40% water. I believe that if Sukanta were to use water content of about 5% he could places his pieces in boxes, ramming the material in and avoiding breakage. I think that the more permeable mold material 5% water would make possible would allow use of the mold at room temperature, so that molds need not be pre-heated to the temperature of the brass. The molds' breakage tends to occur while being moved with tongs from the
kiln to the pouring area.
I think Sukanta may also do well to experiment with a mix of plaster of paris and sand for some of his pieces, but I would value the opinion of others on this list. I have had a bit of success using a fifty fifty mix of plaster and sand for casting glass, but am not certain the technique is viable for brass or bronze. Any insights would be appreciated.
Reid Harvey
Ceramiques d'Afrique, relocated to Bangladesh
Snail Scott on sun 14 jan 01
In my previous note to you, I neglected to mention the
rag-fiber-vs-jute issue. You said that jute fiber was
used, which gives reinforcement, but rag fiber is smaller
and finer. It gives the clay a high plasticity even at
lower water content. So, you could undoubtedly reduce
the water below the 40% figure you mentioned. Cellini
never mentions preheating his molds before pouring, so
it shouldn't be necessary, even though it is standard
practice nowadays. Mainly it's done to prevent 'cold-
pour' problems, not for mold strength. (Cellini did fire
his molds, though, before pouring; they weren't used
'green'.) I wasn't sure if this was the case from your
description.
-Snail
Louis Katz on sun 14 jan 01
Excuse me if I haven't been following the whole thread.
If you pour hot bronze, brass, lead, tin or other molten metal into a clay or
plaster mold that has not been heated to remove both chemical and physical water
you can expect the molten metal to errupt from the mold violently as the water
turns to steam. You MUST heat the mold first.
That said, I will try to go back and read the thread.
Louis
Snail Scott wrote:
> In my previous note to you, I neglected to mention the
> rag-fiber-vs-jute issue. You said that jute fiber was
> used, which gives reinforcement, but rag fiber is smaller
> and finer. It gives the clay a high plasticity even at
> lower water content. So, you could undoubtedly reduce
> the water below the 40% figure you mentioned. Cellini
> never mentions preheating his molds before pouring, so
> it shouldn't be necessary, even though it is standard
> practice nowadays. Mainly it's done to prevent 'cold-
> pour' problems, not for mold strength. (Cellini did fire
> his molds, though, before pouring; they weren't used
> 'green'.) I wasn't sure if this was the case from your
> description.
> -Snail
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Snail Scott on sun 14 jan 01
At 01:18 PM 1/14/01 -0800, you wrote:
>Excuse me if I haven't been following the whole thread.
>
>If you pour hot bronze, brass, lead, tin or other molten metal into a clay or
>plaster mold that has not been heated to remove both chemical and physical
water
>you can expect the molten metal to errupt from the mold violently as the
water
>turns to steam. You MUST heat the mold first.
>That said, I will try to go back and read the thread.
>Louis
Louis-
My original post to Reid Harvey in Bangladesh was sent
directly, that's why there was no 'thread' to follow.
It was my carelessness that put my follow-up onto the
Clayart list inadvertantly. Sorry. (I've appended my
original response at the bottom of this message, but I
didn't save the original question.)
The usual foundry practice with modern ceramic slurry
inventments is to melt out the wax in a kiln or
autoclave. The ceramic slurry usually fuses at these
temperatures. Then, when the metal is to be poured, (a
day or even a month later) the investment mold is
reheated to temperatures which allow the metal to flow
freely without 'freezing off'. The chemical moisture
has already been driven off. Any humdity which has
reentered the investment is driven off in the re-heat.
The discussion with the Bangladesh folks was regarding
'primitive' methods, though. Although I don't know the
precise nature of their process, it seemed to me to be
similar to methods described by Cellini. So, I took
the liberty of describing Cellini's solution to a similar
cracked-mold problem, which involved mixing rotted rag
fiber into the clay investment. From my understanding
of historical practice, these clay molds were fired
first to drive out the wax and harden them, and then,
just before pouring, to drive off any accumulated humidity.
The process generally used was rather time-consuming,
however, since mold then had to be buried in a pit,
so that the metal could flow by gravity from the
furnace to the mold, and the mold had to by carefully
surrounded by earth to support it in the pit. From
this I conclude that the mold could not have actually
been warm by the time the metal was poured, but the moisture
which may have been reabsorbed from the atmosphere and the
surrounding earth was not sufficient to destroy the mold.
Sorry if I was unclear; I didn't mean to imply that the
mold should be unfired, but only that I was making an
assumption that they (in Balgladesh) did fire theirs!)
Here's the post I sent to Reid Harvey directly:
Reid-
I worked for many years in an art foundry. I have also done
some reading regarding historical (low-tech) practices.
Benvenuto Cellini's classic "Treatise on Goldsmithing and
Sculpture" (written in the Renaissance) has good descriptions
of his foundry practices. (It's available as a cheap paperback.)
Most of the book is chatty anecdotes, but there's a discussion
of the best composition for investment material. The clay was
reinforced with fiber made from pulverized and rotted rags.
(I think this was the first 'paperclay' - no different from
using cotton linters, afer all.) The fiber gives the clay
much better 'green' strength. Other fiber sources (nylon fiber,
fiberglass) would probably work, but it counds like the
traditionalmethod (rotted natural-fiber rags) might suit your
situation better.(Cheap, and no supply import problems.) I hope
this suggestion is of some use to you.
-Snail
Folow-up response:
In my previous note to you, I neglected to mention the
rag-fiber-vs-jute issue. You said that jute fiber was
used, which gives reinforcement, but rag fiber is smaller
and finer. It gives the clay a high plasticity even at
lower water content. So, you could undoubtedly reduce
the water below the 40% figure you mentioned. Cellini
never mentions preheating his molds before pouring, so
it shouldn't be necessary, even though it is standard
practice nowadays. Mainly it's done to prevent 'cold-
pour' problems, not for mold strength. (Cellini did fire
his molds, though, before pouring; they weren't used
'green'.) I wasn't sure if this was the case from your
description.
-Snail
Louis Katz on sun 14 jan 01
Dear Reid,
It probably would be best if you could visit the foundries in Thailand in Pitsanulok at Dr. Tawee's Museum, across the river from Bangkok in Thonburi, and about ten miles from Suwanee Natewongs house in Dankwean Korat.
The larger two and three meter molds in Thonburi, plaster clay and sand ,( I believe everything is a secret) are reinforced with jute and steel rebar (3/8" or so). These are not moved but poured in place. The wax drain hole in the bottom I believe is packed with sand, but I have not witnessed this. Other molds are just rolled over and poured in place. The rebar is very reusuable, no need to buy it new either.
I don't know the situation there in Bangladesh but I wonder if it is possible to pour with less moving of the hot molds. I also wonder if the molds are not being burned at too low a temperature. They have to hold up to the thermal shock as well as handling. In Thailand they burn molds in temperary kilns using rice hulls and some charcoal or wood.
If you do go to a plaster mix the investment is reusable so long as it is burned below 1200 F or so. I don't have an exact number. Common practice in the is to reuse this investment and add some fresh plaster.
I cannot say I spent a great deal of time at the bronze factories nor can I be sure of my recollections from there. If you are going through Bangkok and need directions let me know.
Louis
Reid Harvey wrote:
> Fellow Clayartists,
> Yesterday I had the opportunity to visit the workshop of Mr. Sukanta Banik, whose brass casting, lost wax workshop is the last of an 800 year long tradition here in Bangladesh. Mass produced items are encroaching on Sukanta's market, and he needs help in improving his molds, in order to see that there be any chance of the tradition's continuity. (Sorry I have no pictures to share, since my digital camera is having mysterious problems.)
>
> Sukanta has four wax sculptors who are the only one's left in the country who can fashion the bees wax into the sculptures/ models, used for the functional and religious pieces made in the workshop. But one serious problem is that the larger molds frequently break during casting. This means that long hours of work can be made fruitless. This problem with the molds is largely due to their being a bit thin and fragile, since they must be light enough to move from the kiln at the time of casting.
>
> I believe that one related problem is that water content of the mold material used (50% clay, 40% sand and 10% jute fiber) is much to wet at about 40% water. I believe that if Sukanta were to use water content of about 5% he could places his pieces in boxes, ramming the material in and avoiding breakage. I think that the more permeable mold material 5% water would make possible would allow use of the mold at room temperature, so that molds need not be pre-heated to the temperature of the brass. The molds' breakage tends to occur while being moved with tongs from the
> kiln to the pouring area.
>
> I think Sukanta may also do well to experiment with a mix of plaster of paris and sand for some of his pieces, but I would value the opinion of others on this list. I have had a bit of success using a fifty fifty mix of plaster and sand for casting glass, but am not certain the technique is viable for brass or bronze. Any insights would be appreciated.
> Reid Harvey
> Ceramiques d'Afrique, relocated to Bangladesh
>
> ______________________________________________________________________________
> Send postings to clayart@lsv.ceramics.org
>
> You may look at the archives for the list or change your subscription
> settings from http://www.ceramics.org/clayart/
>
> Moderator of the list is Mel Jacobson who may be reached at melpots@pclink.com.
Mike Gordon on sun 14 jan 01
Hi,
The molds should be HOT before pouring any molten metal into them. If
the molds are splitting fron the pressure of the metal pouring in too
strongly or the mold is a week mixture to begin with the molds can be
packed in a wooden box with sand and a mixture of old motor oil to make
it compact and tamp it around the molds this will keep the flashing to a
minimum, which can be taken off with a cold chisle.Mike Gordon
| |
|
