Friday, February 04, 2011

Bronze Casting - Metals, Melts and other Mysteries

I am trying to recreate Viking bronze casting and have been having some trouble. I bought the “ancient bronze” from Rio Grande. It has a melting point of c. 1800 degrees and I have not been able to get it to melt using charcoal and bellows in the little furnace I have made. The pages that I have seen on the web all seem to mention bronze with a melting temp of between 800-1200 degrees. Since you seem to be successful casting in bronze using period methods, can you make any suggestions? Where do you get your bronze? Do you mix your own?

The melting and further the best temperature for fluidity will vary considerably depending on the exact alloy you are using. You might find the MatWeb site helpful:

Its on the technical side, but what you can do is search for bronze, and it will let you see the various commercial alloys with their properties.

My own experience with true bronze alloys (so thats in the range of 90% copper and 10% tin) is that their melting points are in the range you state - so roughly 1800 F

First thing - When someone is quoting 800 degrees, I expect those are 'not Americans' and using Celsius. (Remember at this point the USA is pretty much the only nation solidly still using Fahrenheit. On line converters are our friends) Although 800 C is only about 1500 F, and a bit low for bronze - go to point number two.

Second thing - Sometimes (especially with commercial sources) there is confusion (often intentional) between BRONZE and BRASS. Brass is a lower copper alloy, with less expensive zinc making up as much as 60% of the content. It has a lower melting point. (It also is evil, zinc fumes are extremely toxic and should be avoided! If you see a pronounced blue flames in the torch flame, get volumes of white smoke coming off the surface - just get the heck away from it!)

It should be noted that working with even open topped crucibles in a charcoal fire is unlikely to result in this specific problem. I say this cautioning against repeated exposure to fumes, most certainly avoiding working inside small enclosed spaces with poor ventilation. (Yes - THEY did it, but they also went blind / went crazy / died at 40!)

In the Viking Age, the alloy actually used varied considerably. The primary element was copper, but various amounts of tin, lead and zinc were the other primary components. Lead will lower the melting point and make the material more fluid (in both cases easier to pour). Zinc was not added intentionally, but was a natural component of some ores. From the Roman period onwards, old objects were cut up and re-melted, the resulting loss in volume being made up by adding other metals (typically lead). There is a noticeable shift over time in alloy contents because of this process. Readers referring to texts after about the 1980's may have noticed a shift away from the use of the term 'bronze' to the more vague 'copper alloy'. The spread in actual alloy contents in artifacts is the reason why.

Third - You mention Rio Grande. I have dealt with them (primarily for silver purchases) for many years. I have found them quite reputable and dependable. I'm not sure what the situation is in the USA, but here in Canada, any supplier is required to provide you a 'Materials Data Sheet' on your request. This details the elemental contents, physical properties, and in your (teaching) situation any safety hazards. I'm certain if you asked Rio Grande they would be able to provide you with a MDS on the 'antique bronze' they are supplying.

Fourth - When you purchase 'antique bronze' it is a trade name, not an exact alloy mix. I suspect (see below!) that the exact content will vary both with individual supplier and also likely over time. To that end, I'd recommend you contact Rio Grande (again, they are quite helpful there) and get them to tell you the exact mix and properties of their specific alloy.

A Cautionary Tale:

I worked for a couple of years as a casting technician (dental alloys). I've done a fair amount of fine metals casting for jewellery (lost wax investment casting in silver and gold) and a huge amount of work with the high tin 'pewter' alloys, primarily in stone molds. (This ideal for the classroom, but thats another conversation.) I've also worked with green sand casting using bronze for small objects. In addition, at least messed around with using all Viking Age equipments for casting bronze in clay molds.

Now, you asked where I was purchasing my own alloys.
At the start, I was using scrap. The problem with scrap is that you never know what the heck it *really* is. This can be critical with any metals, but especially anything you are intending to bring to high temperatures. At the melting point, sometimes if there is a lower temperature metal in the alloy, that element can vent off as vapour. Many metal vapours are extremely toxic!

The case in point with bronze is that the heavy metals lead and cadmium are commonly alloyed in with the copper. Both are extremely bad for you. My friend Mike Cardiff (Magic Badger Ironworks) at one point had pretty much destroyed his liver through working (primarily just forging) scrap bronze bars that were laced with cadmium.
Knowing this, I started either using more 'recycled' than 'scrap' materials. That is, metals with a known alloy content. I also started making up my own bronze alloy here. Obviously with the forges and industrial torch sets, I had the temperatures required. I work a fair amount with 'roofing copper' (which is pretty much pure copper, MDS list at 99%), so I just save my scraps and melt them down. (This is not particularly easy, as the copper material has a listed melting point at roughly 1100 C / 2000 F) You first bring the copper up to its melting point. To this I add a measured amount of 'Britannia Metal 92', my lead free pewter / high tin alloy. (Usually at about 10%. There is a small amount of antimony, 1.5 % of this pewter component.)

Anyway, a couple of years back, I was going to teach my first bunch of students a course on 'bronze casting in green sand'. I decided to play it safe (??) and purchase commercial bronze casting alloy, rather than use home made materials. I was a bit pressed for time, so purchased what was called 'antique bronze' from Lacy and Company in Toronto. Now, I had purchased Lacy's material in the past, and had good results with it. I even had some remaining, though not enough for the course.
I should have suspected something was up right from the get go. The new metal was a different colour.
Taking the torch to it produced that warning bright blue tinge to the flame. Once it started to melt, bellowing clouds of a white coloured smoke. I shut down the torch and got everyone out of the room.
At lunch time, thinking it was a problem with my technique, I repeated the steps with my friend and gold/silver smith Brenda Roy watching. It turned out one of my students was a materials scientist for a government lab. Both absolutely agreed with me the problem was with the material.
It was not bronze at all - but (cheaper) high zinc brass.
This was just at the time about five years back when copper prices (thank the Chinese) went astronomically high. They doubled, almost tripled, in mere weeks. Suppliers would not hold quoted prices longer than 24 hours. Lacy's of course does not *make* their 'antique bronze' alloy. They purchase from an industrial supplier and re-package for sales. Curiously, they could not provide me with the mandated MDS when I called. After much yelling and threats of government intervention, they eventually refunded my money on the returned metal. Obviously Lacy's themselves had been scammed. They had purchased what was supposed to be 90% copper bronze, but had been sent 40% copper brass. Their supplier had substituted - and scooped the 50% difference in raw cost.

After than, I just went back to always making my own bronze from my own copper. At least I know what the heck is in it.


Now, you mentioned looking at internet sources, specifically related to Viking Age furnaces.
By far the best, both in terms of experience and knowledge is the work of Anders Söderberg. Anders supports two web sites:
The first is a historic overview of Norse bronze casting.
The second is a more practical description of his experimental work.
As I said, we (being DARC) have messed around with using all norse tools and methods. Maybe a half dozen times. Frankly, although we have accumulated some experience, its more just enough to illustrate the problems and show us what individual elements of the process we still have to work the bugs out of.

There is no doubt that hardwood charcoal, provided with bellows air, will produce temperatures high enough to melt the bronze. Its what the ancients did! The key is in the design of the furnace. Here I refer you back to Anders.
The design of your bellows is important for ease of work and good performance. Here I invite readers to simply use the search function here under 'bellows'. That topic has been dealt with over a large number of past postings!

Original artifact, 'lead alloy' disk broach, cast in stone (?) mold
My replica, high tin alloy cast in carved soapstone mold
Bronze copy, cast in green sand using pewter as master
(all about life sized, 1/4" grid)

5 comments:

STAG said...

Nice article on this topic in "British Archaeology", Oct. 2010 on Viking Jewelry. Part of their article asking "where are the female Vikings?" Most of the jewelry found through female grave goods that are recovered seem to be lead or lead alloy.

http://www.britarch.ac.uk/ba/ba115/feat1.shtml

Anonymous said...

Thanks, good info!

Rhen Nicey said...

Wow! it's a great post! Also, bronze alloys have a desirable and unusual property to expand slightly prior to their setting. This helps a great deal in filling every corner of the mold. Their ductility and strength causes them to be molded into any form.

Anonymous said...

Beware Centigrade (Celsius) v Fahrenheit. Most of the world uses Centigrade and report melting points in *CENTIGRADE*! I've just been looking for the MP of Bronze (in Centigrade, I don't DO Fahrenheit any more) and the temperature range given is as you say. If you are trying to melt bronze at that temperature in FAHRENHEIT you *are* going bto have trouble. Why not join the rest of the world? Here in the UK we changed over without much head-scratching. I changed over 50+ years ago in my teens.

the Wareham Forge said...

Celsius vs Fahrenheit
I am part of the transition generation in Canada. (I actually was part of the lab rat group in Peterborough Ontario growing up - repeatedly had Metric introduced and replaced from about grade 1 onwards.)
As an artisan blacksmith - the units used in the forge are still all primarily in British units (Fahrenheit / gauge / inches).
In practical terms, temperatures are all determined relatively by colour!

Science has almost always been measured in Metric units for me. This applies to archaeology and artifacts.

Just to muddy the waters - a lot of my reader base are Americans - who virtually alone still cling to their own specific units. (How many litres in an *American* gallon??)

Do be aware that the introductory statement was part of an e-mail sent to me by someone else. I certainly knew the '1800 degrees' given was intended to represent *Fahrenheit* measurements.
You see the rest of my post does mark each temperature given with the unit of measurment.

Darrell

 

February 15 - May 15, 2012 : Supported by a Crafts Projects - Creation and Development Grant

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