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Bronze and Brass

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Bronze and Brass

Post by Incognito »

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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Last edited by Incognito on Wed Nov 06, 2019 10:20 am, edited 1 time in total.

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