Glass
Make a transparent, chemically inert, formable solid.
Also listed under Shelter & Architecture › Building materials, because that subject depends on it.
Everything for this step
- Field guide
- Properties of materials → 2 sections Chemistry → 9 sections
- Builds
- Bead microscope · Refracting telescope
- Reference sheets
- None.
- Track
- Ceramics and glass, level 4 of 4
Why this matters
Glass returns more per furnace than almost anything else: windows (which change how long you can work and how healthy buildings are), bottles for storage, and — most importantly — laboratory apparatus and lenses. No glass means no real chemistry, no microscope, no telescope, and no optics.
The shape of it
- Get the batch right: roughly 70% clean silica sand, 15% soda ash or potash as a flux to lower the melting point, and 10% lime as a stabiliser. Without lime the glass is water-soluble — literally. With too little flux you cannot reach melting temperature.
- Wash the sand. Iron contamination gives green glass. That is fine for bottles and useless for optics.
- Build a proper furnace with a refractory crucible, using the firebrick skills. This is a longer, hotter hold than pottery and the crucible is the usual point of failure.
- Melt, then fine. Hold the melt hot for hours to let bubbles rise out. Rushing this gives seedy glass full of bubbles.
- Blow, cast or draw. Blowing gives vessels and — by blowing a cylinder, cutting it and flattening it — window glass. Casting on a flat bed with rollers gives plate.
- Anneal, or lose everything. Cool slowly in a controlled oven over hours or days. Glass cooled fast has internal stress and will crack, sometimes days later, sometimes catastrophically. Annealing is not optional and it destroys more early glasswork than any other error.
- Then make laboratory glass — tubes, flasks, retorts, condensers — which is what actually unlocks chemistry. See A Handbook of Laboratory Glass-Blowing.
- Then lenses. Grinding and polishing a lens gives spectacles (which restore years of productive work to skilled older people), the microscope (which makes germ theory visible and undeniable) and the telescope.
Doing it, step by step
The summary above says what has to happen. This says how, with the numbers that decide whether it works.
1 · Choose and wash the sand
Use clean, white silica sand. Iron gives green glass — fine for bottles, useless for optics. Test candidate sands by melting a small sample with flux in a crucible and looking at the colour of the bead.
Wash the sand repeatedly in water, pouring off the clay and silt, and sieve it to an even fine grain. Keep the purest source for optical and laboratory glass and use the rest for bottles and windows.
2 · Weigh the batch, and never leave out the lime
About 70% silica sand, 15% soda ash or potash, 10% lime, by weight, with the balance made up of crushed waste glass (cullet) — often 20–30% of the charge — which melts first and helps the rest follow.
The flux lowers the melting point; without lime the glass is water-soluble, literally, and slowly dissolves or weathers. Too little flux and the furnace cannot reach melting temperature. Weigh every batch and record it against the result, and mix thoroughly dry before charging.
3 · Build the furnace and a crucible that survives
The melt needs roughly 1,400–1,500 °C held for hours — a longer, hotter hold than pottery, reached with a strong draught and good fuel. Build the furnace from firebrick.
The crucible is the usual point of failure. Make it of fireclay tempered with grog, dry it for weeks to months, and heat it up slowly over days before the first melt. A crucible that fails at temperature loses the melt and can destroy the furnace.
4 · Melt, then fine for hours
Charge the batch in stages as each portion melts down. When it is all liquid, hold it hot for hours — fining — to let bubbles rise out. The melt is ready when a gather drawn out on an iron rod shows few or no bubbles when cooled and held to the light.
Rushing this gives seedy glass full of bubbles, which is weak and useless for anything optical. Skim scum off the surface before working.
5 · Blow, cast or draw at working heat
Glass is worked at around 1,000 °C, where it is thick like honey, and reheated whenever it stiffens.
- Blowing: a hollow iron blowpipe about 1.2–1.5 m long, gather on the end, blow a small bubble, turn constantly. Vessels come from here.
- Window glass: blow a long cylinder, cut off the ends, split it lengthwise and flatten it in a reheating oven.
- Plate: cast onto a flat iron bed and roll.
- Tubing: gather, blow a small bubble, and have a second worker walk away with the other end, drawing it into a tube.
Protect eyes from the furnace glare — years of staring into it cause cataracts.
6 · Anneal, or lose everything
Glass cooled fast carries internal stress and cracks — sometimes days later, sometimes explosively. Annealing destroys more early glasswork by its absence than any other error.
Move every finished piece straight into an annealing oven at around 500–550 °C, hold it there until its temperature is even through, then cool very slowly — over hours for small pieces and a day or more for thick ones — down to room temperature. Never open the oven early.
Test: look at the piece between two sheets of polarising material, or two reflecting glass surfaces at an angle — stressed glass shows coloured bands; well-annealed glass shows none.
7 · Make laboratory glass — the actual prize
Tubes, flasks, retorts and condensers are what unlock chemistry. Draw tubing from a clean melt and anneal it. Work it in a blowlamp flame: bend, seal, join and blow bulbs. See A Handbook of Laboratory Glass-Blowing.
Ordinary soda-lime glass cracks under sudden temperature change: heat laboratory glass slowly and evenly, never put cold liquid into hot glass, and use thin, even walls, which survive thermal shock far better than thick ones.
8 · Grind lenses, starting with spectacles
Grind a lens blank against a curved iron or stone tool with successively finer abrasives — coarse sand, fine sand, emery — washing thoroughly between grades, then polish on a pitch lap with rouge until clear.
Start with reading spectacles, which restore years of skilled work to older people. Near vision fades predictably with age: roughly +1.0 dioptre around 45, +2.0 around 55, +2.5 to +3.0 after 60. Test the strength with the sun: the distance at which a lens focuses sunlight to a point is its focal length, and dioptres = 1 ÷ focal length in metres — a +2 lens focuses at 50 cm. Then the microscope for germ theory.
Build it
Dimensioned drawings, bills of materials and construction procedures for the things this step asks for.