Extractive Metallurgy & the Smelting of Bronze

INTRODUCTION:

Seven metals were in use before the invention of writing: gold, silver, copper, iron, mercury, tin & lead. Many of these are so scarce they couldn’t be used practically (like gold). Others are rarely found in their elemental form, instead found as minerals of carbonates, oxides, or sulfides. It is the elemental form of metals that we are familiar with when we speak of their shiny, malleable and conducting properties. As minerals or ores, metals exist in combination with other elements, often as brittle, non-conducting substances (imagine rocks, salts & crystals). To produce a metal in its elemental form from an ore requires a chemical transformation, a process of extractive metallurgy called smelting. The fundamental problem of smelting involves the chemical reduction of metallic compounds to elemental metals, with different compounds requiring different treatment conditions. Oxide or carbonate ores are heated with charcoal, which combines with the oxygen in the ore, to release carbon dioxide and produce the elemental metal.

Cu2CO3(OH)2 + C → 2 Cu + 2 CO2 + H2O (1)

SnO2 + C → Sn + CO2 (2) Copper was the earliest metal to come into common usage because its ores are fairly common, like that of malachite (Cu2CO3(OH)2), and because it can be smelted at moderate temperatures (see equation 1 above). It is a soft metal, however, which makes it of marginal value for tools and weapons. Tin forms an alloy with copper, called bronze, which is harder than either metal alone. By 3,000 BC bronze had become the dominant metal, so much so that its use defines the Bronze Age. The higher temperatures required for iron production delayed the advent of the Iron Age to about 1200 BC.

Tin metal is not found in its elemental form in nature, but it ore, cassiterite (SnO2), was known (see equation 2). Thus, malachite and cassiterite must be smelted together to make the alloy, bronze. Neither malachite nor cassiterite melt at low temperatures. A flux must be added to get these two compounds to melt. A flux is a compound that melts at relatively low temperatures and in doing so acts as a solvent to dissolve the other reactants. Chemical reactions do not take place in the solid phase because they do not come in close contact (most of the reactant is locked on the inside of macroscopic lumps). The liquid phase brings them in close contact to facilitate the reaction. In this experiment you will smelt malachite and cassiterite using corn kernels as a reducing agent and soda ash (Na2CO3) as a flux to melt the reactants. You will carry out this reaction in one of the ceramic crucible you made earlier in the course.

PROCEDURE:

This experiment will take two lab periods. On the first day you will mix the reagents, add them to your crucible, and seal the vessel. The vessels will then be fired in the electric kiln to cone 05. On the second day you will retrieve your bronze, polish it, and record its weight.

1. Weigh out about 10 g copper (II) carbonate basic in a plastic weigh boat. Note: It does not have to be exactly 10 g. Whatever mass you use, be sure to record it here. Mass of Cu2CO3(OH)2:______grams 2. Transfer this copper ore to a plastic ziplock bag. 3. Weigh out approximately 2.0 g of tin (IV) oxide in a plastic weigh boat & transfer it to the same plastic bag. Record actual weight here: Mass of SnO2:______grams 4. Weigh out about 6 g sodium carbonate in a plastic weigh boat & transfer to the plastic ziplock with the other reagents. Mass of Na2CO3:______grams 5. Seal the bag and mix the reagents thoroughly being sure to break up any lumps. 6. Pour the reactants into your crucible. 7. Weigh out about 10 g of corn meal in a plastic weigh boat. Mass of corn:______grams 8. Place the corn kernels in a neat layer on top of the other ingredients in the crucible.

Smelting is a reduction process and as such, oxygen must be excluded. For this reason, the crucible must be closed to prevent oxygen from oxidizing the metal.

9. Using a fresh piece of clay, roll it into the shape of a snake and place it on the mouth of the crucible. Place the lid on the crucible, compressing the fresh clay to make a tight seal. Then trim away any excess clay with a spatula and smooth it out with your thumb.

Your vessel will be fired to cone 05 (approximately 1888°F).

10. When you get your crucible back during the next lab period, write down any observations. 11. Carefully pry out the lump of bronze at the bottom. 12. Clean up the bronze with fine sandpaper.

Experiment adapted from:

Dunn, K. M. Caveman Chemistry, 1ST ed.; Universal Publishers: 2003, 109-120.