Optical-quality glass is made by melting a small batch of 1-2 kg with extra care, holding it at high temperature and stirring repeatedly to drive out bubbles and streaks, pouring it into a preheated mould and cooling it at 1 °C per hour through the annealing range. It must be free of bubbles, free of striae (compositional streaks) and free of strain. Stirring and a long high-temperature hold are safer routes than toxic fining agents.
Part of Glass Blowing and Forming
Melting and cooling glass so that a lens can be ground from it.
Optical Glass Fails On Three Defects That Each Have A Separate Cure
The carrier lists three requirements and one method. The corpus’s Bubble-Free Glass article adds the reasoning.
| Defect | Cause | Cure |
|---|---|---|
| Bubbles | Dissolved gas, trapped air, reboil | Longer hold at high temperature; stir |
| Striae | Uneven composition | Stir; homogenise by stirring and remelting |
| Strain | Fast or uneven cooling | Very slow anneal |
What You Need
- Pure sand and flux, finely ground so the batch melts evenly; see Silica Sources.
- A crucible and furnace that hold the melt at the maximum temperature for hours; see Glass Melting.
- A stirring rod that withstands the melt. The glass-melting article uses a preheated iron rod; iron in the melt tints the glass, so a fireclay rod is worth trying for a colourless result (recalled, not from the corpus).
- A preheated mould to receive the pour; see Mold Construction.
- A slow-cooling oven; see Annealing Oven.
Method: Melting For Optics
Step 1: Batch 1-2 kg, the carrier’s size. Small batches are easier to hold uniform.
Step 2: Melt, fine and condition as in the glass-melting article, but hold at the highest temperature the furnace sustains. The bubble-free-glass article gives 4-8 hours or longer for the fining hold, and 6-10 hours in its small-scale procedure; the glass-melting article gives 1-3 hours for ordinary glass.
Step 3: Stir repeatedly at high temperature.
Step 4: Lower the temperature slowly to the working range without reheating rapidly, which can reinject gas.
Step 5: Pour into a preheated mould.
Step 6: Cool at 1 °C per hour through the annealing range.
Step 7: Inspect for bubbles and streaks by looking through the glass against a bright sky; and check strain between crossed polarisers.
Worked Example
The carrier gives 1 °C per hour. At that rate the 100 °C span used in the lehr schedule (hold, then cool to 100 °C below) takes 100 ÷ 1 = 100 hours, about 4.2 days. The lens-blank article asks for “24+ hours”: at 1 °C/h, 24 hours covers 24 °C. Either 24 hours is a minimum rather than the full schedule, or the annealing range being slow-cooled is only about 24 °C wide; the corpus does not say which. Plan the longer figure.
Common Mistakes
- Fining with arsenic oxide. The bubble-free-glass article calls it highly toxic; the long hot hold works without it.
- Reheating quickly after fining, which can bring bubbles back.
- Pouring into a cold mould, which chills and strains the glass.
Safety
Hot glass does not look hot
A poured block, and the mould around it, fall below about 600 °C and look cold while far hotter than a hand can bear. The slow-cooled oven hold at 500 °C or so is invisible in daylight.
Silica dust and toxic fining fumes
Finely ground sand for batch makes the worst silica dust: it scars the lung permanently. Wet the batch, mix outdoors and wear the best respirator available. Arsenic and antimony oxides, used as fining agents in some recipes, are poisonous and give off dangerous fumes when heated.
What’s Next
- Learn next: Pressing Lens Blanks — turning the glass into discs
- Combine with: Stress Testing — checking the anneal