Slag Baubles

February 14, 2008

Last weekend we hiked in the hills south of our house, where grass, cactus, and thorny shrubs give way to agaves and desert oaks.  A hundred years ago, there were several active copper mines in the area, and while hiking we see glory holes, ore piles, old dirt roads, and a shiny black heap of slag that looks like a small mountain of obsidian.  I can imagine what it must have looked like at night, through the dusky coal smoke of the smelter – the molten metal glowing white, then darkening to red as the copper bars cooled; the fiery orange slag splashing onto the pile, reeking of sulfur – until one night around 1910, when the inevitable happened and a forest fire destroyed the smelter, and gave the land back to the yuccas and oaks.  But we have the collector’s instinct that drew the first miners here, and we pick through the slag and bring home a treasure trove of tiny glass drips that look like bones, twigs, or strange machine parts.  Some may find their way into jewelry, but I’ll just put most of them in a small copper bowl.

copper smelter slag drips

4 Responses to “Slag Baubles”

  1. JJ ColourArt said

    Twenty-six years ago my husband and I found ourselves stopped in a mining town with a huge slag heap by the bus station.

    We rooted through it and I found the “Slag Heap Madonna” as I call her, because the slag looks like a woman in a long dress holding a baby. She is sitting on my computer desk here.

    It is like looking at clouds. Slag–what is it, what’s the story?

  2. ironwing said

    When the ore is heated, it “roasts” or oxidizes, then melts. The heavy molten metal sinks and is allowed to run off. The rest of the melt is poured out as slag, which is a waste product. Typically it contains a lot of silica, with some iron oxides, sulfur, etc. Once it is poured out, it cools very quickly and forms a glass. As it flows and cools, it may develop lava-like patterns and structures similar to those seen in natural volcanic glass. Essentially, slag is man-made obsidian. Iron gives it the black, dark green, or dark red colors. The pieces in the photo above haven’t been cleaned yet, and are still dusted with brown iron oxide and powdery yellow sulfur. The ore that was mined in this area is metallic golden chalcopyrite (copper-iron sulfide), dark red cuprite (copper oxide), and various blue-green copper minerals (malachite, chrysocolla, etc.).

  3. JJ ColourArt said

    I’m surprised it isn’t made more often into jewellery. If it contains a lot of silica it must polish well.

    Thanks for the particulars on slag and the colouring. Mine came from a nickel mine in Sudbury, Ontario and is a reddish colour with black.

  4. ironwing said

    For the last few years, a couple of dealers at the Tucson gem shows have had cabochons of “Swedish Blue” slag. They are banded or swirled in shades of dusty bluish-turquoise. Some are quite attractive, but man-made materials are a hard sell at the Tucson shows where there is so much natural stuff competing for people’s attention.

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