Templated tube growth about a reactive gas plume, the pharaoh’s serpent reaction

The pharaoh’s serpent reaction, the thermodecomposition of mercury thiocyanate in air, is a classic chemical demonstration. We analyse and characterize the reaction products, composed of two different phases, graphitic carbon nitride, C3N4 and metacinnabar, HgS. We show that two growth modes are involved in forming the sinuous solid product; the more common one, jack-up growth, forms a solid cylinder from the base up. This is accompanied by a rarer second mode, chimney growth, in which fluid mechanics plays a fundamental role. In this second growth mode, reagents are transported in gaseous form in a reactive plume to be precipitated remotely on the growing rim of a tube templated around the plume. Such templated tube growth, while common in a liquid environment, for example in the chemical-garden reaction, has not been reported in a gaseous medium.