In order to make glass marbles, rods of coloured glass are needed, and are melted together to form a sphere. First, a glass rod should be selected that is approximately 6mm in diameter. The tip should be heated gently (to prevent it from breaking under the sudden change in temperature), and then more glass should gradually be added. Whichever rod colours are chosen, the term for this slowly growing piece of glass is a 'gather' (see glossary).

In order to make the marbles a spherical shape, a graphite mould in a convex shape (or, half a sphere hollowed out) with a handle is to be used. To make the marbles rounded, the hot gather is place in the mould and the rod spun between the thumb and the first, second, or third fingers, in a smooth back and forth rhythm.

All the time during which the gather is in the heat, it must be ensured that it is neither too hot, nor too cool. Keeping the glass in the blue part of the flame, sustaining an orange-red glow will assure this. Should the glass become too hot, it will flare (sometimes called a 'soda flare'). This will mean that the glass will turn a bright glowing yellow. It can even turn a glowing white, and begin to drip or run.

When enough glass has been gathered to produce a marble of the desired size, a 'punty' (see glossary) can be attached, on the opposite side of the glass sphere from the main rod, enabling the glass to be pulled and twisted at the same time. The punty is then removed, and the glass reshaped so that it is smoothly spherical.

Once the marble is again a nicely rounded sphere, the marble should be transferred to the punty. To do this, the tip of the punty and the area on which is it to be attached is heated. It must be ensured that one is cooler than the other. The marble should be glowing a bright orange, while the punty glows a cooler red. Alternatively, after heating both of the pieces, the tip of the punty can be tapped on a 'marver' (see glossary). Doing so will ensure that the punty is cooler than the marble.

By holding the punty and giving it a sharp tap, the marble should fall off the punty and into the mould. The 'pontil' (see glossary) should be quickly fire polished, before the marble is turned out of the mould in between two ceramic fibre blankets. And hour or two should pass before the marble it looked at, otherwise it will be ruined through thermal shock - the cooler air in the room will cause the glass to break. Sometimes this may happen anyway within a day, a month, or sometimes years, when the stress can cause the marble to break in pieces. Putting the marble between the blankets too hot will leave marks on the marble, whereas putting it in too cool will cause it to break. It should be blanketed just as the glow wears off.

Glossary:

Fire polishing
The reintroduction of a vessel into the glory hole (A hole in the side of a glass furnace, used to reheat glass that is being fashioned or decorated. The glory hole is also used to fire-polish cast glass to remove imperfec tions remaining from the mold.) to melt the surface and eliminated superficial irregularity or dullness.

Gather
(Noun) A mass of molten glass (sometimes called a gob) collected on the end of a blowpipe, pontil, or gathering iron, (verb) to collect molted glass on the end of a tool.

Marver
A smooth, flat surface, on which softened glass is rolled when attached to the blowpipe or pontil in order to smooth it or to consolidate applied decoration.

Pontil, pontil mark
The pontil, or punty, is a solid metal rod that is usually tipped with a wad of hot glass, then applied to the base of a vessel to hold it during manufacture. It often leaves an irregular or ring-shaped scar on the base when removed. This is called the "pontil mark."

References:

How To Make Glass Marbles - http://www.artglass2.com/glass_marbles.html

A Glossary of Glassmaking Terms - http://www.cmog.org/Education/glgloss.htm