In the typical 19th century wood-burning iron stove the size of the hotplates was adjustable by a set of rings. The stove – and its rings – was entirely made in blackened cast iron, an ideal material. Blackened sand-cast iron is still an ideal material to place hot cooking pots on, heat enduring and forgiving for heat and grease stains.
The inner plate rests in the outer ring. If you need a second trivet lift the plate aside and you have two, one trivet for the frying pan and one for the saucepan. The way the sand-casting is made it produces different surface structures on the up and down sides. The two parts of the Flat Iron are turned diametrically, telling the tale of the casting process and indicating that they can be separated.