|Table of Contents for Caveman Chemistry: 28 Projects, from the Creation of Fire to the Production of Plastics|
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The rabble has long since lost interest in this book. Content to be buffeted about by a world they neither apprehend nor comprehend, they close their eyes and hope for the best. Their hands are pale, their fingernails clean, and their minds are filled with spirits who demand little and offer nothing at all. You will forgive me, my brothers and sisters, if I am circumspect but what I have to tell you is of the utmost importance and sensitivity. I mean to speak to you of life and death.
I know that you feel a sympathy for the mortal you inhabit. You may even feel that you are that mortal, but nothing could be further from the truth. The mortal is a creature of the moment, with passions and appetites which see no further than tomorrow or next month or next year because it lives for only a century, at best. Your life, distinct from that of the mortal in which you currently reside, is potentially much longer and your vision must look beyond the concerns of the moment. Looking back on history, the great movements and advances have taken place across centuries and millennia, time periods too vast to hold the attention of a mere mortal.
If we are not mortals, then what are we? We are the essence of humanity, not the substance; we are the spirit, not the soul; we are the mercury, not the sulfur; we are the element, not the compound. Are you confused? If so, then perhaps you are ready for a great lesson. What follows is not a proof of our separate existence; it is but a physical metaphor to help you understand the circumstances in which we live.
You will use the crucible you made in Chapter 5. The crucible must be sound and free of cracks or weaknesses. It must have a conical interior to funnel molten materials to the center and it must have a tight-fitting lid. Your crucible must have been previously fired, or bisqued, to prove that it can keep in the heat and withstand it. You will fill your crucible with the ores of the metals you wish to smelt.
Copper melts at 1083°C, which is at the upper range of the temperatures which earthenware clays can withstand. Rather than move to a more refractory clay, the cassiterite adds tin, which lowers the melting point and produces a harder metal than copper alone. There is no "correct" amount of tin to be added; bronze is a solution, an alloy of copper and tin, not a compound, and so its composition is variable, not fixed. You may make bronze with 10% tin, 20% tin, 50% tin, or whatever you choose. The melting point and hardness of the bronze depends on the concentration of tin, just as the strength of an alcoholic beverage depends on the concentration of alcohol. Bronze for casting bells is typically 20% tin, that for casting cannons, 10%. Both malachite and cassiterite may be obtained from a pottery supply as copper carbonate and tin oxide, respectively.
There is a minor dilemma in making bronze; bronze melts at a lower temperature than copper, but it cannot form until the copper and tin melt and mix. In practice, a flux is added, a material with a melting point lower than the materials to be melted. In this case, you will add either potash or soda ash, materials discussed in Chapter 8. You may use potash you made yourself, or you may purchase "washing soda" and dry it in an oven at 130°C (270°F) for an hour to convert it into soda ash. There is no "correct" amount of soda ash; you simply need enough to cover the malachite and cassiterite with molten slag. For a first bronze you may use 10.0 g of copper carbonate, 2.0 g of tin oxide, and 6.0 g of sodium carbonate. Weigh these materials on a centigram balance using the methods of Appendix C, mix them in a plastic bag, and place them into your crucible.
Finally, we need a reducing agent, which will combine with the oxygen in the ores and leave as carbon dioxide. Charcoal was the reducing agent of choice in antiquity, being inexpensive, plentiful, easy to make from wood but rather messy. You may use dry seed corn, which turns to charcoal in the kiln without making a mess of your work-space. Place the corn on top of your mixed ores, nearly filling the crucible but leaving enough space that the lid makes a tight fit.
Smelting is a reduction process and as such, oxygen must be excluded. To that end, the crucible must be closed to prevent oxygen from oxidizing your metal. Using a fresh piece of clay, roll a snake and place it on the mouth of the crucible to form an ouroboros, a snake biting its own tail. Place the lid on the crucible, compressing the ouroboros to make a tight seal, as shown in Figure 9-1. Then trim away any excess clay with a knife or spatula and smooth it out with your thumb.
Equation 9-1 shows that gas is produced during smelting and you must provide for this bejeezus to escape. The ouroboros will make a good seal between the crucible and its lid but it will not "glue" them together. As gas builds up in the crucible it will lift the lid slightly and escape. In a sense, the ouroboros serves the same purpose in smelting that the fermentation lock did in brewing.
The smelting crucible must be dried before it can be fired. You may either put it in an oven at 130°C (270°F) for an hour or set the kiln to include a drying segment before the firing. Fire the crucible to cone 05. When the firing is complete and the crucible cool, remove the lid and explore the interior. If there is no charcoal then your lid did not seal properly or there was insufficient corn from the start. You are unlikely to find any metal. If there is charcoal present, remove it and explore the charred material beneath. There may be beads of metal adhering to the walls of the crucible; if so then the atmosphere within was sufficiently reducing to smelt the ores into metal. Continue to explore the bottom of the crucible with a knife or spatula. If the ores contained sufficient metal, if the atmosphere was sufficiently reducing, if the shape of the crucible was sufficiently conical, and if there was sufficient flux present then you may find a large nugget of bronze in the bottom, as shown in Figure 9-2. Carefully pry it free from the crucible and scratch the blackened surface to reveal the golden crop beneath. Clean it with fine sandpaper or a wire brush and polish it to a high luster.
What have you learned from this operation? Molten rock from within the Earth separated, congealed, and re-melted, the elements separating and recombining to produce myriad rocks and minerals. Some were exposed by wind and water; others were buried and crushed by the weight of the overbearing rock. The rock has changed form many times over the eons, from sulfides to sulfates to carbonates to oxides. It was dug out of the ground, crushed, milled, sorted and separated, and came to you as blue and white chalky powders. You placed them into a crucible along with other obviously non-metallic materials, and yet when you opened your fired crucible, a nugget of metal lay in among the cinders. The metals were there all along! In the same way that the metals passed from one mineral to another over the ages, so you have passed from one mortal to another. Do not believe that you are salt, my brothers and sisters, mere mortals of flesh and blood and bone. Do not believe that you are sulfur, the life which animates the body. No, you are a mercurial spirit, the essence of humanity, and the mortal is merely your temporary home.
You placed obviously non-metallic materials into a sealed crucible of your own making. After firing it, you recovered a nugget of solid metal, a talisman to remind you of your noble identity. You will record in your notebook the weight of copper carbonate and tin oxide used in your smelting as well as the weight of the resulting bronze nugget. You will also include a photograph of your nugget as a record of your achievement.