スズ石からスズを抽出する — スズ鉱石のパニングと製錬

錫石（SnO₂）は二酸化スズであり、スズの主要鉱石です。モース硬度は6〜7、比重は6.8〜7.1と非常に高く（一般的な砂の2.6よりはるかに重い）、色は黒から赤褐色まで変化します。高い密度により、金のパンニングと同様に、重力選鉱（パンニング）で容易に分離できます。

錫石は、含スズ花崗岩の下流にある沖積層（河川）堆積物中に、重い黒色または暗褐色の粒として見つかります。川の砂利の中、重い鉱物が集まる川の曲がりの内側、大きな岩の背後を探してください。歴史的なスズ産地には、コーンウォール（英国）、東南アジア、ボリビア、中央アフリカがあります。

Shovel gravel and sand from locations where heavy minerals accumulate: bedrock crevices, behind boulders, and the inside of river bends. Fill a bucket with raw gravel. The cassiterite grains are small (1-5 mm) and mixed with ordinary sand, quartz, and other minerals.

Use a gold pan or wide shallow bowl. Add a scoop of gravel and fill halfway with water. Swirl and tilt to wash away light material. Heavy cassiterite grains sink to the bottom center. Continue washing until only black heavy grains remain. This is identical to gold panning technique.

Cassiterite grains are: black to dark reddish-brown, very heavy (noticeably heavier than quartz), have a brilliant adamantine (diamond-like) luster when scratched, and a white to pale brown streak. Magnetite (another heavy black mineral) is magnetic — test with a magnet to separate it from cassiterite.

Repeat panning until you have a concentrate of mostly cassiterite. Use the magnet to remove any magnetite grains. You need at least 500 g of clean cassiterite concentrate for a worthwhile smelt. From 20 kg of gravel in a good tin-bearing stream, expect 100-500 g of concentrate.

Spread the cassiterite concentrate on a flat surface in the sun to dry completely. Wet ore in a hot crucible causes dangerous steam explosions. Drying takes 2-4 hours in direct sunlight. The dry concentrate should feel gritty and pour freely.

Use a clay or graphite crucible that can withstand 1200°C. Mix the dry cassiterite with an equal volume of powdered charcoal. The charcoal provides carbon for reduction: SnO₂ + 2C → Sn + 2CO₂. Tin melts at only 232°C, far below the smelting temperature, so it collects as liquid at the bottom.

Place the loaded crucible in a charcoal forge with bellows. Tin smelting requires lower temperature than copper or iron — about 1000-1200°C is sufficient for reduction. Pack charcoal around and above the crucible. Put on safety equipment: leather apron, face shield, leather gloves.

Pump the bellows to bring the forge to operating temperature. Maintain heat for 1-2 hours. The cassiterite reduces to metallic tin which, being liquid above 232°C, pools at the bottom of the crucible below a layer of slag and ash. Tin smelting is one of the easiest metal reductions.

Using crucible tongs, lift the crucible from the forge. Carefully pour the contents into a preheated sand or stone mold. The heavy molten tin pours first (bottom of crucible), followed by lighter slag. Tin is silvery-white when molten and solidifies quickly due to its low melting point.

Tin solidifies within minutes. Once cool, remove the ingot from the mold. The tin ingot will be covered with a thin layer of slag — break this off by tapping. Clean tin is silvery-white and very soft — you can scratch it with a fingernail (Mohs hardness 1.5).

Tin has distinctive properties: bending a tin bar produces an audible crackling sound called 'tin cry' caused by crystal twinning. It is very soft (scratched by a coin), silvery-white, and does not rust. When heated, it melts easily at 232°C — far below a candle flame temperature.

Weigh the clean tin ingot. From 500 g of good cassiterite concentrate, expect 300-350 g of metallic tin (60-70% yield). Record the source location, concentrate weight, smelt time, and tin yield. This tin is ready for alloying with copper to make bronze, or for tinplating, soldering, and pewterwork.