Bell Casting

On Sat, 16 Oct 2004 12:13:55 +0100, Andy Dingley


Exactly! Visit my website: http://www.frugalmachinist.com Opinions expressed are those of my wife, I had no input whatsoever. Remove "nospam" from email addy.
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Any sand will work, but finer the better. Silica is typical (sandblasting sand should work, though it could be finer) but olivine has become popular, though more expensive.
Any clay will work, stickier (fine-grained like bentonite) or refractory (ball clay) the better. Fireclay is very refractory but not generally very sticky. Bentonite has historically been the primary medium.
I happen to use ball clay because that's all I can get at the local ceramic store. My sand is standard masonry sand, 50 pounds a bag from Home Despot. Pretty rough but that's what files are for... I recently mixed a batch of maybe 80 grit sandblasting sand and haven't tried it yet, but should work nicely.
Lay out the sand in a pile, toss on a few pounds of clay powder (don't bother with kitty litter products, it's been fired and as such loses most of its stickiness without extremely fine milling), mix it with a shovel and moisten. You'll know when it's right... it sticks together without leaving moisture on your hand, feels kinda soft when you break it (if it crushes suddenly, it needs more clay) and isn't excessively sticky (too much clay).
Too much clay and/or water reduces porosity and increases gas and can lead to steam voids in or on the casting, or even worse, bubbling of the metal.
Read up on sand casting (check my website and a slew of others on the webring and ABYMC links pages) and start with white metals, then aluminum, then try bronze. Just some nonsense castings, plaques or something for instance. Then try molding the bell.
You can also attempt it in lost wax, but you lose your pattern if you screw up. (You can make copies with the help of a silicone mold, but you need gallons of RTV and that's a lot of dollarsigns.) Here you take maybe 2 parts sand and 1 part plaster of paris, mix it with water and fill in around the pattern. Then fire in a kiln to 1200 degrees F over a few hours until it is uniformly heated and burned out. Then you can pour metal in it (mind that aluminum will cool VERY slowly if you pour into a mold this temperature).
Tim
-- "I've got more trophies than Wayne Gretsky and the Pope combined!" - Homer Simpson Website @ http://webpages.charter.net/dawill/tmoranwms
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[I think you need to do more homework, Eric. I seriously doubt that Paul Revere used the sand-casting process to produce bells. It was only developed in the 19th century, and really didn't become common until the 20th, with the invention of matchplates and bentonite-bonded sands. It's much more likely that he used the traditional lost wax method that's still being used (with some modifications) to make large bells.]

[Horse manure doesn't make sand stick together. But it is used to temper the clay used in traditional lost-wax casting techniques - here's a link describing how some people tried to recreate ancient casting methods: http://www.geocities.com/zozergames/bronze2.html . If you don't want to use it, sawdust or powdered charcoal can be substituted.]

[While there are plenty of resources on sandcasting, I really don't think this is a historically correct approach, even without the kitty litter. You might as well use ceramic shell. Here are a few links to historic bell-foundries, describing how they do it. They mention that this is the way it's always been done....]
http://www.russianbells.com/founding/bellmaking.html
http://www.verdin.com/casting_bells.htm#casting
http://www.eijsbouts.com/moulding.htm
[By the way, I came across this note in my research - is this Gellomer/Gillimore an ancestor? He probably shared the usual technique known to bell-founders at the time...]
Bell Founding (http://www.whitman-ma.gov/history1.html )
"In 1769", says Benjamin Hobart, in his History of Abington (1866, p140-1) " a deserter from the Bristol Army, a bell founder by the name of Gillimore, was employed by Colonel Aaron Hobart". The town records on 8 January 1770 of Old Abington give his name as Thomas Tallamore, and records of Old North Bridgewater ( now Brockton) spell his name Gellomer. The reader can take his choice. Mr. Hobart says that his father sent representatives to Boston to teach Paul Revere to mould and cast bells, this being the beginning of the famous Revere bells that are now collector's items.
Andrew Werby
www.unitedartworks.com
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Andrew Werby wrote:

" a deserter from the Bristol Army, a bell founder by the name of

That's neat, but we can't claim any close relation. At that time, our branch was in Yadkin County, North Carolina, by way of Barbados. No doubt on the run from something else.
Kevin Gallimore
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Kevin,
Your past relatives may have been on the run but they must have left an impression on the good folks of Yadkinville as there is a Gallimore Dairy Rd. just as you cross into the county. :-)
Rusty Bates
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Rusty Bates wrote:

That's because they don't know about cousin Elwood....
Kevin Gallimore
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And on the board of one of the major US zinc smelting companies.
I hear he got another job later on though.
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Sand casting is old technology. Sand casting has been done for hundreds, if not thousands of years.
Paul Revere was a silversmith as his main craft and did later turn to bell casting. As I read and understand your post your main goal is to cast bronze bells. Bells have been cast in bronze for hundreds of years also, probably even back to ancient Chinese who were some of the first foundrymen. "Lost Wax" or Investment casting is ONE technique used to make bells but only a small percentage of all bells cast throughout history. I would estimate 95% of all bronze bells ever cast were cast in sand.
For molding in sand there are two main items to consider:
1) The base (unbonded) sand and what you will use for bonding.
The sand selections can be silica sand (for example) which will range in fineness from AFS GFN 35 to 120 or even higher. These are numbers that describe the average size of individual sand grains. The numbers DO NOT relate to the same sizes of grain on sandpaper. The "AFS" stands for American Foundrymen's Society, and the "GFN" stands for Grain Fineness Number. Lower number = coarse sand and higher number = fine sand. Coarser sand leaves more "perm" (short for permeability) to the sand which allows gases to escape easier in the casting process. Finer sand will reproduce detail more easily. For a starting project I would recommend about AFS GFN 70. Also you might encounter the term "3-screen" or "4-screen" sand. This refers to the distribution of the sand. In other words the average would be "70" but there would be a certain percentage of 40, 50, 70, 100 and 140 sized grains in the mix. Also sand may be round grained or angular shaped. This describes what the grains will look like under a microscope. In normal situations round grain works best. Sometimes for special applications we will use angular shaped grains. There are also other types of sand, besides silica sand, such as chromite, zircon, olivine, and other minerals. At the foundry where I work, we use AFS GFN 38, 3 screen, for the chemical bonded foundries and AFS GFN 70 for the "green sand" foundry. We also use an AFS GFN 65 lake blend, and AFS GFN 55 chromite sand, and occassionally AFS GFN 70 zirconium sand.
2) What will be used for bonding.
Green sand means a sand that has clay in it for bonding. Water activates the clay. You might use about 3-4% clay and about 1-2% moisture. You will have to experiment a bit. You might try to find a local foundry who will sell you some of their sand already mixed, if you decide to use green sand. There are more modern methods you might use to bond your sand. If you are making just a few molds, and can afford to throw out your sand, I recommend using an air-set or chemical bonded sand. Your results will be much more likely to succeed on the first try. There are many standard chmicals used to make chemical bonded sands. For example there is the phenolic-urethane process, the furan process, the sodium-silicate process, and many others. While the sand can be recycled, and used over, after teh mold is poured and the casting shaken out, you have to re-crush the sand back to grain fineness level to be able to recycle it. A big advantage of chemical bonded sand is that the mold surface which is in contact with the molten metal can be coated with a refractory. This is like a high temperature paint. It will give a nice surface finish so the polishing of the bell can be done easily, if you want a shiny bell.
A few years ago Glenn Ashmore showed how the casting process was done to make a huge lead keel. You might find his website on line with a decription of how it's done. At the foundry where I work we use the furan process, to cast parts weighing from a few pounds to 30+ tons. We coat the sand mold molds with a water-based coating and dry it. The surface finish is exceptional. We reclaim 97% of the sand with ball mills to recrush the sand for use all over again. In Ohio, from 2001 to 2003, the Verdin Company set up a mobile foundry. They cast a 300 pound bronze bell in the county seat of every Ohio county (88 in all). The molds were made in phenolic-urethane, and coated with a refractory coating of graphite and zircon. They would do the whole operation on a Saturday and Sunday and move to the next county once a week. The world's largest singing bell is in Newport, Ky. In 1998 a group a very philanthropic businessman in Newport Kentucky commissioned the casting of the World Peace Bell. This huge bronze bell was cast in France, brought to the U.S., and rung for the first time at the change from 1999 to 2000. It was called the Millenium Bell. I was involved in the early stages of the Verdin Mobile foundry and Millenium Bell projects.

Already answered above, but try chemical bonded sand if your making just a few but use green sand if you're willing to keep it going as a long term project. Or try both.

That would not have worked as described. The horse dung would have been a carbon product and I doubt if it had any "glue-like" properties, even if you threw it against the wall to see if it would stick LOL. But, they might have added small amounts of horse dung to act as a carbon additive to the sand. While pouring the molten metal into a green sand (clay bonded) mold, the carbon products will form a soot-like gas which automatically coats the interface between the metal and sand, with a thin layer. This formed layer prevents the molten metal from easily "burning-in" to the sand. What foundries use today are "seacoal" which is finely ground coal, or other carbon type products. I have seen wood flour, crushed corn cobs, and other materials used as well. Most likely the people in Paul Revere's foundry used a natural bonded sand. This would be a sand that might have come from a river bank, and contained a lot of clay in the sand. The problem would have been that this sand would have contained an excess of free water (i.e. water not being utilized by the clay). During pouring the free water turns to steam, and "blowholes" and a fair amount of steam can develop. Even a slight excess of free water will cause burn-in on the casting so the carbon in the sand (about 1/4 to 1% is plenty) would offset any free water.

I doubt they gave you the full details of what they are doing.

The clay is bentonite. There are two types, Western and Southern. Southern works great for thin section metals, and lower melting point casting, because that clay develops a lot of strength and you will find it retains its shape easier. Western has better hot strength. You can blend the two clays to customize a mix that works best. If you're making say 10 pound bells 100% Southern bentonite would work OK. You could add a little western if you have access to it but it's not important unless you're pouring larger castings.

You've only scrathed the surface of what you need to know. Please ask more questions and I think you will find this list will provide the answers you need.
Mark
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Wow. That's the kind of response that makes this group famous. Thanks to everyone for their tips, especially those of you who have taken the time to put together carefully illustrated guides to help newbies.
First off, I figure I should clarify a mistake, since we've now done some more of our homework.
Paul Revere did not use sand casting to make bells, although we think he did do sand casting. I'll check up on that and report back. The technique was a variation of the lost-wax method. I'll give you the authoritative statement on it: "Bell makers first created an inner 'core' that modeled the inside diameter of the bell. They accomplished this by digging a hole in the ground, building a hollow pile of bricks in the center, covering them in a special mud, and using a pattern to pack the mud into the shape of the interior of the bell. Second they applied a mixture of tallow and wax to the outside of the core, creating a perfect wax model of the bell. Workmen added any lettering or designs to the wax at this point... Third, workers applied numberous thick coatings of 'bell mud' on top of the wax, creating the 'shell,' which was a model of the outer surface of the bell. When the shell hardened, a workman lit a fire inside the hollow brick structure at the center of the core, which melted the wax and allowed it to drain off, hardening the core in the process.... The shell was then hardened with additional fire, and covered in sand or loose soil to prevent it from bursting. Finally, molten metal was poured into the space between the core and the shell." - from "Paul Revere's Metallurgical Ride: Craft and Proto-Industry in Early America" by Dr. Robert Martello, which happens to be the Ph.D. dissertation of one of our teachers for the course. (Yeah, good thing we found that.)
Also, we took a look at a translation of De Pirotechnia, which was conveniently located in the reserve section of the library. It's pretty interesting, but really long. So, we have one person reading that and reporting back. I think I got the better end of the deal by offering to post to the newsgroup.
As far as the actual casting is concerned, in the interest of time, we've decided to order some Petrobond. Fortunately, we already have some lying around (but not enough to cast the bells), so we're going to try casting white metal on Tuesday. The MatSci professor has some experience with this, so combined with what we've read, we think we can do it without hurting ourselves.
We still have a good deal to work on, and I'll probably be back with more questions about molding shortly.
Thanks again,
Eric Gallimore
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Eric G wrote:

Once the wax was melted out, what kept the outer mold from shifting relative to the inner core? I'm sure you would have a connection at the bottom (with holes to allow the wax to drain), but would it be strong enough to hold when the molten metal is poured into the mold?
Also, would the metal have to be poured into a hot mold to allow it to flow, or is there wax residue, a carbon layer, or something to facilitate the pour?
Just wondering.
Joe
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Joe wrote:

The assumption is simply the center is held by the central stack up core. The outside is being held out by the outer stack up. Simply, the outer stack up must be strong to hold it's own weight and to be able to span the distance.
Think of a coffee cup over something else. The cup doesn't crush down when their is air space between.
Martin
--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer snipped-for-privacy@pacbell.net
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Wow. That's the kind of response that makes this group famous. Thanks to everyone for their tips, especially those of you who have taken the time to put together carefully illustrated guides to help newbies.
First off, I figure I should clarify a mistake, since we've now done some more of our homework.
Paul Revere did not use sand casting to make bells, although we think he did do sand casting. I'll check up on that and report back. The technique was a variation of the lost-wax method. I'll give you the authoritative statement on it: "Bell makers first created an inner 'core' that modeled the inside diameter of the bell. They accomplished this by digging a hole in the ground, building a hollow pile of bricks in the center, covering them in a special mud, and using a pattern to pack the mud into the shape of the interior of the bell. Second they applied a mixture of tallow and wax to the outside of the core, creating a perfect wax model of the bell. Workmen added any lettering or designs to the wax at this point... Third, workers applied numberous thick coatings of 'bell mud' on top of the wax, creating the 'shell,' which was a model of the outer surface of the bell. When the shell hardened, a workman lit a fire inside the hollow brick structure at the center of the core, which melted the wax and allowed it to drain off, hardening the core in the process.... The shell was then hardened with additional fire, and covered in sand or loose soil to prevent it from bursting. Finally, molten metal was poured into the space between the core and the shell." - from "Paul Revere's Metallurgical Ride: Craft and Proto-Industry in Early America" by Dr. Robert Martello, which happens to be the Ph.D. dissertation of one of our teachers for the course. (Yeah, good thing we found that.)
Also, we took a look at a translation of De Pirotechnia, which was conveniently located in the reserve section of the library. It's pretty interesting, but really long. So, we have one person reading that and reporting back. I think I got the better end of the deal by offering to post to the newsgroup.
As far as the actual casting is concerned, in the interest of time, we've decided to order some Petrobond. Fortunately, we already have some lying around (but not enough to cast the bells), so we're going to try casting white metal on Tuesday. The MatSci professor has some experience with this, so combined with what we've read, we think we can do it without hurting ourselves.
We still have a good deal to work on, and I'll probably be back with more questions about molding shortly.
Thanks again,
Eric Gallimore
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THe horse dung was almost certainly not used for sand casting but for a ceramic mould casting, for which dung was frequently used throughout history.
Check out "on Divers Arts" which specifically has information on bell casting, and "De Re Metallica". Both are available from Dover.
Also Anders Soderbergs web page may be helpfull, http://members.chello.se/vikingbronze/vikingbronze.htm

jk
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