Drinking cups made from clay can become odorous, can contaminate taste, soaks up flavor and are a breeding ground for bacteria. For these reasons it is nice to have actual glass for drinking containers and as we know glass is useful for so many other things as well, such as windows, eye glasses, binoculars etc.!
The steps to make glass are relatively simple; however, it is easier to say it than to actually do it. Why? Because high temperatures are required to make glass. Sand melts at 3002F. The addition of other ingredients brings the melting temperature of the sand down to 1600 degrees Fahrenheit. Luckily you can use charcoal to reach temperatures a little bit hotter than this.
So how do you make the glass? Since sand melts at a very high temperature we need a way to bring this melting temperature down. Luckily flux can do this. Flux is a material that brings down the melting point of another material. For our purposes we will use potash
as the flux.
Alternatively broken glass in place of sand if it is available. Some glass will begin to get soft at around 1100-1200 degrees and go liquid at around 1400. But to make glass from sand requires a much higher heat for a longer period of time along with other materials besides sand.
However, this glass is not permanent and water will eat away at it over time. Luckily calcium in limestone used as an ingredient in the glass making process will stabilize and make the glass last virtually forever... or until you break it, whichever comes first!
Using these three ingredients, sand, potash and limestone we have the ingredients for glass. They should all be ground up to a powder and mixed. The ratio's would be 60% sand, 30% potash and 10% lime. It will take days to achieve a melt.
Still, reaching the required melting point is not as simple as building an open fire and melting your sand into glass. To reach high enough temperatures to melt sand you will need a kiln aka furnace and a good fuel source.
when supplied with forced air will produce the required temperatures.
To actually make the glass the three ingredients are placed in a clay pottery
crucible. The crucible is placed on a ledge inside a kiln and a fire is lit. Eventually the fire turns the sand/potash/limestone into glass. When cooled down you have a glass ingot.
Link: How to build a kiln aka furnace. Kiln