Bronze and Brass

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Incognito
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Bronze and Brass

Post by Incognito » Mon Nov 04, 2019 2:37 am

I expect most people know the difference between bronze and brass, for those who don't here is a short explanation. Not everyone keeps to definitions and you might find them muddled sometimes, but bronze is an alloy of copper and tin, brass of copper and zinc. Tin is a soft metal that when combined with copper restructures the lattice in copper making it harder and tougher, and eventually more brittle. Zinc is a more brittle metal that does the same but needs greater amounts to achieve this and the resulting brass seems to be missing some of the toughness of bronze. Sometimes both are added to copper, sometimes with other elements also, but we will keep it simple here with straight alloys.

So standard bronze is somewhere around ten percent tin, if you go much higher it starts to get very brittle. That property can be used to make bell material. Also ancient mirrors used high tin bronze because it approaches a silver colour and is very reflective.

Brass is cheaper ( because zinc is cheaper and makes up a larger part of the metal) . Below 37% zinc it remains quite malleable, then hardens off.

Both resist corrosion, higher percent brass is maybe more susceptible because zinc is less stable. Both have been used since ancient times, brass less so and I think partly because zinc was less available, not included by direct addition but by combining ores.

So here is a basic chart of samples I prepared, and as noted elsewhere bronze stands out as more reddish. Only 10 % zinc brass looks at all reddish, which could feasibly be confused with 20% bronze, except 10% brass is not common. 30% bronze is going on silver coloured, and the 40% brass is not well lit in the photo but is basically a light yellow. All the brasses except 10% are distinctly yellow. So, the differences are still subtle, and more so if the light shines a different way on bronze, but it is still possible to distinguish a reddish tone to it even then.

I also include phase diagrams here. They are not as complex as they look. From left to right you have percentage of metal. The very top liquidus line marks the temperature the metal is fully liquid. Sometimes underneath that there is the solidus line, underneath which the metal is solid. In between the two the metal is half liquid. The rest of the boxes just mark changes in lattice structure when cooler or cold, i.e. all arranged evenly, or crystals of one surrounded by the other etc. Those are not too important for this short explanation. The normal range of alloys used is coloured below the chart, bronze has two areas, standard bronze around 10% tin, and bell metal bronze which is a little over 20% tin.



In the photo the very center is copper. In the diagrams Cu is copper, Zn is zinc, Sn is tin.
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Re: Bronze and Brass

Post by fred » Mon Nov 04, 2019 5:26 am

Cheers. ::g

It took me donkeys years to understand what the grotty bits of scrap that I find were made of. In practical terms as long as you can sort your scrap into copper and brass you should get half decent rates for it from the scrap merchant. I've never yet been challenged on whether any of my brass is actually bronze. :D
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Re: Bronze and Brass

Post by Rayc » Mon Nov 04, 2019 7:58 am

A lot of information here in a few words. Thanks.
I think the composition of Tombac buttons (up to 20% zinc) is indicated in the top diagram. Or am I wrong?
In field archaeology things are kept simple - artefacts are all described as 'copper alloy' before they are sent off for post-excavation analysis.....and if funding is short that description stays.

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Re: Bronze and Brass

Post by Incognito » Mon Nov 04, 2019 11:14 am

fred wrote:
Mon Nov 04, 2019 5:26 am
Cheers. ::g

It took me donkeys years to understand what the grotty bits of scrap that I find were made of. In practical terms as long as you can sort your scrap into copper and brass you should get half decent rates for it from the scrap merchant. I've never yet been challenged on whether any of my brass is actually bronze. :D
Bronze is worth more in theory for having more copper, you could always thin down bronze to brassier to scrape a penny or two, just need to buy or make a furnace and throw in some pot metal with the scraps.

(Probably not worth the effort and cost though :D)

Just looked at prices, brass is 2150 ton, bronze 2400 so no wonder they didn't challenge you. Going to need a mighty big furnace to turn the ton of bronze into some kind of brass though, probably get arrested into the bargain for starting volcanoes or something. Not really economical I guess :(
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Re: Bronze and Brass

Post by Incognito » Mon Nov 04, 2019 11:43 am

Rayc wrote:
Mon Nov 04, 2019 7:58 am
A lot of information here in a few words. Thanks.
I think the composition of Tombac buttons (up to 20% zinc) is indicated in the top diagram. Or am I wrong?
In field archaeology things are kept simple - artefacts are all described as 'copper alloy' before they are sent off for post-excavation analysis.....and if funding is short that description stays.
Thanks. All the information is a swine to find, searching didn't even bring me a palette of brass and bronze samples for different % of metal, which is why I made my own and why I posted it here along with a condensed description of the alloys. On the other hand online searching takes unexpected directions and you end up learning about all sorts, it's just funny that the basics don't show up in a concise presentation. Zinc oxidises rapidly with heat, flares up with a blue flame when added to molten copper, so smelting it at temperature required I expect is more recent and beforehand they had to to add ores that liberated some when smelting copper. Few copper alloys are pure, previously you would usually find smaller or larger percentages of other metals mixed in, but the above is the main basis of what is found regarding copper ( the others being alloyed with silver and with gold ) . So copper alloy is a catch all where to start analysing exact composition is costly or difficult. Usually though, with a bit of experience, you can make a pretty close guess by just looking at the surface, and even closer with a few simple tests.

I'm not familiar with the term tombac, will look it up. Yes 20% it is in the top diagram to the left.. Everything in the large window above where it says tombac/brass is alpha phase, that is to say it structures like pure copper, and therefore has similar properties (for example is malleable).

I'll include a link in a little while that explains better what phase diagrams represent.

::g

This link has clearer explanation of what the phases represent.

https://www.mah.se/upload/_upload/Phase%20diagram.pdf

You can ignore the complex math, it has lots of pictures that give the basic idea :D I have yet to understand how certain phases at higher temperature carry through to the lower temperature, I think that is for a rainy day. Tombac is listed as low zinc brass, anywhere below 20% zinc, and with or without traces of other metals.

This link describes at the beginning how and when zinc came into use

https://www.insa.nic.in/writereaddata/U ... _Art02.pdf

It is not found in a pure natural state. This link is the closest I came to finding a colour palatte for bronze and brass. It only lists some bronzes and lead bronzes, photos are tucked away at the end.

https://exarc.net/issue-2017-2/ea/colou ... gy-project

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Re: Bronze and Brass

Post by fred » Wed Nov 06, 2019 3:23 am

Rayc wrote:
Mon Nov 04, 2019 7:58 am
A lot of information here in a few words. Thanks.
I think the composition of Tombac buttons (up to 20% zinc) is indicated in the top diagram. Or am I wrong?
In field archaeology things are kept simple - artefacts are all described as 'copper alloy' before they are sent off for post-excavation analysis.....and if funding is short that description stays.

You can tell by the condition that they come out of the ground in that the composition of tombac buttons varies considerably too. ::g
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