This panel shows the making of zinc metal-spelter. The 'furnace' is a battery of zinc retorts.  The zinc produced is gaseous, and is condensed in a receiver. The liquid zinc is drawn off at intervals and poured into moulds, as seen in the foreground.
The solid zinc was known as spelter. It was at that time used for coating iron or steel objects - galvanising - to protect against rust, and also formed into sheets, or sections. 
The connection with Maryhill is that David Swan Junior, Glasgow's only spelter make in the 1870s, had his works near the Kelvin Dock on the canal.