What is it?

Glass is an amazing substance. It is a permanent liquid, in that it never quite sets solid.
The flow of glass is so slow that to all intents it behaves as a solid, but given enough time it will slump and flatten!

The most basic glass is produced from melting sand (silica and quartz) - and - wood-ash (alkaline) together, at temperatures of about 1000° C.
To produce colours, you add assorted chemicals and minerals.
Construction of artefact in the Iron Age was limited to coiling and moulding, the technique of blowing glass arrived with the Romans.
Glass in its molten state is thick and sticky not unlike honey, and needs to be held in a crucible in the furnace. Sometimes kilns where used as furnaces, and did two tasks with one firing. Extra tunnels and holes were sometimes made in the sides of kilns, through which the glass could be handled.
Beads were the easiest, a metal rod could be dipped into the molten glass. It was then lifted out of the crucible, and whilst still in the heat, turned round to wind the glass onto the rod. It was then withdrawn from the furnace, still turning it, until the glass cooled sufficiently to be slid off the rod. This produced a bead with a hole through, ready to be strung into a necklace.
The second method was moulding. The runny glass was pushed or run into a mould and kept in place until cool enough to retain its shape.