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The Wind Furnace

This cupellation furnace might have been used by Newton, but is not mentioned in his list of alchemical apparatus. Courtesy Science Museum, London.

Among Sir Isaac Newton’s papers are descriptions of a surprising number of furnaces.  Newton is best-known for the laws of physics which still bear his name; his three laws comprise all the basic materials for a theory of mechanics, or, of the interactions of objects large enough to see.  But physics is not a field that needs furnaces, except maybe for building an experimental apparatus or two.  Rather, furnaces are among the basic equipment of chemistry.  With heat, the raw stuff of the world could be dried, powdered, purified, separated, calcined, and so on; with heat again, the same things could be fused, melted, combined, annealed, or hardened.  In the furnace simple reagents are won from the world; also in the furnace, they are recombined in new and surprising ways.  And it was this field that occupied Newton’s last years; after offering the world a comprehensive theory of physics, then one of optics, he turned his mind to alchemy.  He turned his mind to the work, but also his hands: he himself laid the bricks for the furnaces that would populate his laboratory.

Made in New Jersey: the Motley Lab’s 1970’s era hot-plate/stirrer promises temperatures as high as 550F.

Most of the heating at the motley lab is done on a combination stirrer and hot-plate.  The hot-plate function replaces an apparatus Newton called a sand furnace; this was a dish of sand suspended over coals, which could bring enough heat to boil water.  And the stirrer, I suppose, takes a good-paying guild job away from an apprentice who would have stood there with a rod, stirring.  But the first recipes for Prussian Blue call for reagents extracted from animal coal, which is blood that has been dried in the sun, ground into a powder, and calcined to purge away everything that is not carbon, nitrogen, and iron.  Those first two steps could be done by the apprentice, whose place is taken by me.  But the last one?  That requires the application of heat: extreme heat, for an hour or more.  No hot plate will do; not even a normal fire source, like a bed of charcoal or spirit lamp, proved to be enough.

The team, after completing construction of ye Wind furnace, and preparing it for firing using the bottom-loading method. (Click here to see it in action). Photo for the Motley Emblem.

After discovering that the only “Prussian” color you can get by working with these basic heat sources was the nauseating stew I have tabbed Prussian brown, it became clear that the motley lab was going to need a serious technical upgrade.  It needed what Newton called a “Wind furnace,” a furnace which “blows it self” to astonishing temperatures.  A technology like this is what Gilbert Simondon calls a “concretized” object, a machine that uses a single process to affect multiple things.  The idea is to use the rising exhaust gases of the fire to do two things at once.  As the heated air passes up a narrow chimney, it heats a crucible containing the matter to be calcined; but its rising also pulls fresh air into the combustion chamber, thereby intensifying the heat below.

The wind furnace was a crucial component in eighteenth-century chemical laboratories– including the workshops of the colourmen; using just a small amount of fuel, a small chamber can be raised to surprising temperatures.  A range of different versions existed; William Lewis, whose course in chemistry was published nearly simultaneously with Sterne’s Tristram Shandy, describes about half-a-dozen varieties, including one modular unit which can be stacked in different configurations.  The lab’s wind furnace was built at 1:4 scale according to directions provided by Robert Dossie, in his Handmaid to the Arts (23-25).  Dossie’s is unclear on how it is to be loaded and fired, but contemporary sources suggest at least two different methods.  The first is simply to pack the firing chamber with strawberry-sized lumps of hardwood charcoal, placing the crucible within the coals.  I have also had good results feeding hardwood offcuts in through the lower opening– what Dossie calls the “ash hole”; these transform to charcoal as they burn, without significantly obstructing the flow of air.

The process is reciprocal, or, runaway– at least to a point; the more heat in the combustion chamber, the swifter the rising gases, and the more air is pulled into the firebox to replace it.  Once the process gets going, the firing chamber easily surpasses the limit on the motley lab’s infrared thermometer– which maxes out at 900 degrees Fahrenheit.  However, because the heated air continually brings fresh oxygen through the intake, the entrance to the furnace hardly becomes much warmer than the surrounding environment.  Indeed, since the whole carefully arranged pile of bricks warms up as it works, the coolest spot in the furnace is the little porch just in front of the firebox, where the flames are fed with fresh fuel.  An ethanol lamp, which chymists like Robert Boyle used for simple heating, doesn’t get much hotter than about 500F– which is useful for boiling or evaporating, drying or fermenting.  But that amount of heat is hardly unpleasant enough to change the molecular structure of a compound.  Those processes, called “calcination” if it divides, “fusion” if it combines, are two of the nine or so processes of early modern chemistry; they can only be done in a wind furnace.  The very word “calcine” comes from the process that transforms limestone (calcium carbonate) into calcium oxide.  At temperatures above about 1500F, the carbon in limestone is burned away, where it is releases as carbon dioxide gas.  What remains is the caustic white powder called quicklime: a reagent in metal manufacturing, a component in concrete, for use in plaster and mining and so on.

The least pleasant place in the furnace, where potassium ferrocyanide is won from animal coal.

You can make “Prussian brown” with dried blood heated over an ethanol lamp.  I know, because that was my first try at the blue.  Prussian brown is pretty much just rehydrated blood with a bunch of other mildly toxic chemicals mixed in.  But Prussian blue requires the extraction of a lattice-like compound formed from potassium, iron, and cyanide, linked in a lattice-like structure.  That is only to be got, at least in eighteenth-century chemistry, by calcination, with heat enough to convert organic matter into ash.  Into the furnace 15 grams of powdered blood ground with 5 grams of pearl ash; what comes out is about 6.5 grams of calcined coal, ready for boiling and filtration.  The rest is purged away.