To press a lens blank, gather the right amount of optical glass while it is still soft and press it between two steel plates into a thick disc. For a 50 mm lens the disc should be about 55 mm in diameter and 15-20 mm thick, which takes roughly 90-120 g of glass, and it is then annealed very slowly for 24 hours or more. The disc is made oversize so the grinding tool has glass to remove.
Part of Glass Blowing and Forming
Forming a thick glass disc that will be ground into a lens.
A Blank Is Pressed Oversize Because Grinding Only Removes Glass
Grinding cannot add glass, so every dimension of the finished lens has to fit inside the blank. The carrier’s figures for a 50 mm lens: a disc about 55 mm in diameter, 15-20 mm thick.
The corpus’s Glass Blank Prep and Lens Grinding articles cut blanks from sheet glass instead; pressing from molten glass is the alternative when you have an optical melt from Optical Quality Glass.
What You Need
- Optical glass at pressing temperature.
- Two steel plates, faces flat and smooth, the lower with a shallow recess or a ring to hold the disc diameter.
- A coating against sticking: the carrier gives lampblack for iron moulds; the same soot coat is the thing to try on steel plates.
- A lever or screw press, or a heavy hand lever, to apply even pressure.
- A preheated annealing oven; see Annealing Oven.
Method: Pressing
Step 1: Gather the right amount of optical glass.
Step 2: Press between the two plates into a thick disc.
Step 3: Check the size: about 55 mm across and 15-20 mm thick for a 50 mm lens.
Step 4: Anneal very slowly, 24 hours or more per the carrier.
Step 5: Inspect for bubbles, chill cracks and strain between crossed polarisers before grinding.
Worked Example
Volume of the disc: area of a 55 mm circle is π × 2.75² = 23.76 cm². At 15 mm thick (1.5 cm) the volume is 23.76 × 1.5 = 35.6 cm³; at 20 mm it is 23.76 × 2.0 = 47.5 cm³. With a density of about 2.5 g/cm³ (recalled, not from the corpus) the glass weighs 35.6 × 2.5 = 89 g to 47.5 × 2.5 = 119 g, so roughly 90-120 g. Take a gather a little heavier to allow for trimming.
The disc is 55 mm for a 50 mm lens: 5 mm larger, which is a 10% margin (55 ÷ 50 = 1.10). The Lens Grinding article asks for a blank at least 20% larger than the finished diameter, which would be 60 mm for a 50 mm lens. The two do not agree; the larger margin is the safer one.
Common Mistakes
- A blank barely larger than the lens. Edge chips take the margin.
- Pressing glass that is too cool. It chills against the plates and cracks.
- Skipping the 24-hour anneal. A strained blank shatters under the grinding tool.
Safety
Hot glass does not look hot
A blank leaving the press falls below about 600 °C within moments and looks cold; the steel plates stay hot too. Move both with tongs.
Pressing hot glass
A hand press has a pinch point, and glass that squeezes out sprays hot fragments. Keep hands clear of the plate edges and wear eye protection and a face shield.
What’s Next
- Learn next: Basic Lens Grinding — from blank to lens
- Combine with: Stress Testing — screening blanks