Making glass


Glass was used a lot during Barton’s time. 
Before the invention of plastics, glass held most liquids. 
Glass was used in making photographic negatives, and vision-aids like magnifying glasses, spectacles, microscopes, and nautical and astronomical telescopes. 
Let’s explore how glass was made way-back-when. 

The first glass was not man-made. When lightning strikes a sandy beach, clumps of glass, called 'fulgurites' are formed. (photo) Also, a few volcanoes in the world make a glass called 'obsidian', which was-is used by tribal people to make arrowheads and spear tips. 

Every tangible thing we have is somehow made from things that have existed since the earth was formed, 
or made during the earth’s lifetime, like coal and oil. 

The basic recipe for glass is:
 
1) Silica sand (SiO2)      (60-75% of the mix) 
2) An alkaline ash, potash (K2CO3) or soda ash (Na2CO3)      (12-18% of the mix) 
3) Calcium Carbonate (CaCO3) from powdered limestone or seashells      (5-12% of the mix) 
4) Other ingredients can be added to affect color, strength, etc. 

The method for making glass is similar to making a metal alloy:
 
1) Pure ingredients of the correct proportion, 
2) in a crucible (pot) that can withstand the heat needed for the process,
3) with enough heat to melt the ingredients. 

Pour in a proper mix of silica, ash and limestone. Use a fine grade of sand. 
The smaller the pieces, the faster they melt. (Ask your science teacher why.) 

Alkaline ash has a lower melting temperature than silica. 
I suspect that as the ash melts, it ‘hugs’ the sand and transfers heat more efficiently. 
Also, if you have seen a vinegar and baking soda volcano, consider that sand is slightly acid and ash is alkaline, 
so when the ash melts, it may also react with the sand on a pH level; chemically, not just thermally. 

Oddly though, alkaline ash makes the glass ‘water-soluble’, so water can dissolve the glass. 
(imagine that; a water glass being water-soluble). 
So, CaCO3 (limestone) is added to reinforce the sand and repair the weakness that the alkaline creates. 

The individual melting points of glass ingredients are: 
Silica sand: (called ‘the Former’, the main ingredient)3110F. 
Potash: 146F (called ‘the Flux’, which lowers the melting point) Soda ash: 1564F 
Calcium Carbonate, limestone or shells: 1517F (called ‘the Modifier’ because it strengthens the glass) 

Total melting point: 2600F – 2800F. 
With silica sand’s melting temp of 3100F, see how adding other ingredients alters the melting point of the main ingredient? Wow. 

So, the melted glass may then be made directly into something, 
like window glass (float glass), 
or bottles or other molded items
or into optical discs, to be ground to proper design later, 
or it can be formed into rods or sheets, for artisans to use off-site in their studios. 

Eric J. Rose 
middlegrademysteries.com 
photo: srmgs.com
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