A month ago when I offered to write this WAYWO I didn’t have anything specific in mind. Interests fill my plate and always have. Some have only been hobbies, some passions and others passing curiosities. I thought through the list of things I’m currently dabbling at and nothing seemed ready for writing up so natural I tossed another iron on the fire. Well, not iron but something just as hard.
Bronze has fascinated me for years. A combination of copper and tin it’s the oldest alloy known to man.1 It has secrets and it’s held onto them.
Once upon a time bronze was used to make mirrors. The Latin word for “mirror” is “speculum.” That’s also the name of the bronze metal used to make early mirrors. By weight, one third tin and two thirds copper. It’s silver in color, highly reflective, heavy, and brittle. When dropped it shatters like glass.
Three thousand years ago a hand sized mirror was the iPhone of its time. Everyone one had to have one and when they broke or became too tarnished to use they were thrown away. Consequently ancient garbage heaps are full of them.
In the late 1400s mirror technology evolved significantly with the invention of tin-mercury amalgam backed glass mirrors. Though the coating was poisonous2 these mirrors were still being manufactured into the early 1900s when they were finally supplanted by silver backed glass. A technique invent by Justus Liebig in 1756.
The first description found in literature of a concave bronze mirror is from 1610 however a concave mirror is pictured in a treatise from 1535 and concave bronze mirrors are found in ancient garbage heaps.
Telescopes, a life long passion, were first made only from lenses but by the late 1600s the science of optics had advanced enough that reflecting telescope design were being devised. The first functional reflecting telescope is attributed to Newton in 16683 though there is evidence that a mirror / lens combination telescope was created a hundred years earlier. Newton would have been familiar with those writings.
A reflecting telescope needs it’s reflecting surface in front. A back surface reflection means the glass distorts the formed image. Tin-mercury amalgam not sandwiched between glass and a lacquer coating didn’t lend itself to that. Something hard, reflective and polish-able was needed. From the late 1600s to the late 1700s reflecting telescope mirrors were made from speculum bronze.
Three weeks ago I 3D printed a plastic model of a 56mm diameter mirror to cast in speculum.
This is not the first time I’ve cast speculum but of the half dozen speculum mirrors I’ve made only one has become part of a telescope.
Most modern telescope mirrors are made from glass or ceramic then coated with aluminum or sliver on their front surface. An extremely thin over-coating of some hard transparent substance is applied to protect the fragile metal coating. A metal mirror has the distinct advantage of not needing a metal coating. There are so many down sides to them though that unless there is a specific need they are rarely used in telescopes.
Speculum bronze hasn’t been used to make telescope mirrors for 300 years and the people who developed the technology took many of their secrets with them to the grave. I read all there is to read of those early workers, experimented and recreated techniques using modern tools. I even gave a talk about it all. I once had great plans to make a twelve inch diameter speculum mirror and was well on my way when the realization of just how difficult and unlikely success would be finally set in.
Two weeks ago I piggy backed another Jewelry class casting and when all the student were done. I plopped my graphite crucible filled with broken pieces of speculum bronze into the melting furnace and soon cast my mirror. I did not have the materials to properly prepare the metal so I expected the casting to be riddled with pits4. While it’s not perfect, as speculum goes it’s not bad either.
Then came the “grinding” and polishing. Glass mirrors are ground by rubbing the glass mirror blank against either another identically sized piece of glass or more often these days a cement disk with porcelain tile epoxied to it. Progressively finer silicon carbide grit is a slurry of water between the two discs is used to grind out the curve. That doesn’t work with speculum or metal in general. Speculum isn’t ground it’s sanded. I printed the model with a curved face but it still needed to be sanded
smooth once cast. The curve I chose I believed would match an old glass grinding tool that naturally when I went to get it I couldn’t find.
Well #%@$, I don’t want to make that again. Wait, I have a 3D printer, I can print a tool and glue sand paper to it. It doesn’t have to stand up to grinding, it just has to be the right shape. I drew the tool in Blender and printed it only to find it was too shallow. I made it more curved and printed it again. Still too shallow. Third times a charm, it was close enough. I plowed through the grits 220, 500, 1500, 2500 all the time worried I would sand through the millimeter thick metal face but I didn’t. I was successful, I had an unpolished mirror. It scattered light everywhere but it was good enough to form an image. In three weeks I’d gone from a drawing in Blender to a telescope mirror that needed polishing out. With glass that’s done with pitch generally poured over the grinding tool and pressed into shape with the finely ground mirror blank after the pitch lap solidifies. A polishing compound in water between the two is used to polish the glass. That doesn’t work with speculum. Well, it doesn’t work well. What does work are polishing pads.
Using the same 3D printed plastic tool I glued a polishing pad to it. When I did this a decade ago I used pitch and cerium dioxide as the polishing compound. It worked but a film of bronze would build up on the pitch eventually spalling and scratching the mirror. Before I started using polishing pads that meant I had to regularly scrape the pitch with a razor blade and then repress it into shape. It was frustrating which is why I tried polishing pads. Except for the initial polish, polishing pads are considered a no-no by telescope makers as they leave a rough surface. A thin layer of bronze still builds up on the polishing pads but it doesn’t so easily lead to scratches.
Now I must make a telescope OTA (Outer Tube Assembly) to stick the mirror in. I’ve drawn the mirror cell and printed part of it. I need to draw up and print a secondary assembly to hold a small prism and a simple eyepiece. A couple of more weeks and I should have a telescope. I’ll present it in the comments of a future WAYWO.
1) https://en.wikipedia.org/wiki/Bronze
2) Little Willie from the mirror licked the mercury right off.
Thinking in his childish err that it would cure the whooping cough.
At his funeral his mother sadly said to Mrs. Brown
“ ’Twas a chilly day for Willie when the mercury went down.”
— Ruthless Rhymes for Heartless Homes by Harry Graham 1901
3) Newton’s telescope mirror was 2 inches in diameter with a 6.25 inch focal length.
4) Speculum like all bronze is prone to porosity. This results from the inverse steam reaction. Dissolved hydrogen and oxygen are rejected when the melted metal solidifies. They react and form steam which leaves tiny bubbles in the now solid metal. This can be mitigated by both de-gassing (hydrogen is called “gas”) and de-oxidizing the melt before casting.
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