(C) Daily Kos This story was originally published by Daily Kos and is unaltered. . . . . . . . . . . WAYWO: Stop Casting Porosity [1] ['Dan K'] Date: 2026-08-02 Amateur Telescope Makers live in a quiet corner of the nerd-o-sphere. We’re a friendly bunch. Here in the Bay Area, the Chabot Science Center Friday Night Telescope Making Workshop brings us together. A few of us are extroverts within our specialties. Visitors drawn in by the sign or lost looking for the observatory’s telescopes get shown around and their questions answered. The people working are generally glad to take a break and talk. Pushing glass is exercise. I’ve been attending again, working my metal mirrors and making sure everyone who stops by knows this technology is obsolete and with good reason. I direct them to people working glass. A few weeks ago, I cast at home, using well-worn equipment and new crucible tongs, two new five-inch diameter speculum blanks. The learning experience was successful. The propane kiln, an almost empty gas canister, and its back up. On the bricks on the left the new tongs. They can be used to both lift the crucible from the kiln and pour the metal. I need to get fire proof covers for the gas hoses. ynohtnA The electric kiln. It is neither older nor as old as me. It just looks that way. On the far right, sticking out from behind the kiln there is half of a white ceramic thermocouple. Accordingly, the temperature meter reads zero. Somewhere there is a replacement. It is safe. ynohtnA The doing was straight forward. Melt the metal while heating the mold. Prepare the metal. Pour the metal. There are of course details, special tools, metallurgical chemistry, leveling the mold, annealing, and don’t trip over the tongs Anthony. Speculum billets in the crucible, in the propane kiln with a little borax flux sprinkled on top of them. ynohtnA A few minutes later the billets are beginning to melt ynohtnA More minutes later the molten metal with some flux floating on the surface ynohtnA Like all bronzes, speculum is prone to porosity, though particularly so. The speculum used to make telescope mirrors from the late 1600s through the mid-1800s exemplified this. The polished surfaces were marred by a myriad of pits. They complained about it. Speculum telescope mirror for use in a Gregorian telescope. The mirror was made by James Nasmyth between 1840 and 1850. In the detail of the Nasmyth mirror extensive pitting can be seen. Attribution: Science Museum Group ynohtnA Porosity lowers effective density. If there’s a big hole in the middle, it’s not going to weigh as much. Using reported sizes and weights of centuries old mirrors held by the Science Museum in South Kensington, London. I calculated density ranging from 8.1 to 8.4 g/cm^3. For comparison, the density of silver is 10.5 g/cm^3, the density of aluminum 2.7 g/cm^3, the density of glass 2.5 g/cm^3. My metal is practically pore free. Modern metallurgical chemistry makes this possible. The densities’ range from 8.8 to 8.9, plus or minus the range. Measuring the density of an irregularly shaped object isn’t easily precise. My density measuring apparatus. The scale is limited to 50g with a measurement uncertainty 0.0003g ynohtnA The first billets I cast thirty years ago, before I knew how to stop casting porosity have a density of around 8.7. I believe the quality of that metal is due to metal purity. The copper and tin I use is 3N+ (99.9+ % pure). The purity of stock copper and tin 200 years ago was at best 90% to 95%. Even so those long-ago makers had everything they needed to significantly reduce the casting porosity, they just didn’t know how, or what. Shortly, the world learned the “what” and heavy metal mirrors were replaced by silvered glass. Speculum was abandoned. Of course there were hold outs for quite a while. Though speculum was inferior in terms of reflectivity, weight, and maintenance, it was easier to sell what was well known with a proven track record than something new with no track record.1 The second 5″ blank in the early stages of hogging (removing the material that makes the mirror concave.) ynohtnA The first blank cast wedged. As it was, it made the most sense to cutoff the worst out it. Which I did, leaving a slightly smaller blank. The second casting was less wedged. Rather than first flattening the casting and then hog out the depression as I’ve done in the past, I started with hogging. Sketching contours as they did, the shrinking surface defects have slowly pin pointed a few small deep crevices. They are not long. To me they are glaringly obvious. I won’t sand through them though. Progressively thinner and thinner mirrors is how I’ve broken so many of them. Broken speculum mirror. The mistake that broke it I knew might happen and how and still I broke it. . Twenty five years ago I alloyed one hundred or so, ~110g speculum billets. One third tin, two thirds copper. Approximately eleven kilograms, 25 pounds. I’ve been picking away at it ever since. With billets, broken bits, and unalloyed copper and tin there is enough metal to cast a full thickness2 6-inch speculum mirror blank, 9 lbs ( a little more than 4kg). I have yet to cast metal so riddled with pores its density was under 8.7. I’ve tried. I’m trying. If they can screw up that bad, so can I. During the technological transition, had google existed the trending questions might have been: What is silvered glass? When did speculum drop out? Can I change my purchase? Full thickness refers to the ratio of mirror diameter to thickness, specifically 6:1. A six inch diameter mirror would be one inch thick, a twelve inch, two inches thick. This last Friday and the workshop there was a 16” full thickness mirror blank being worked. It weighed 50lbs. A full thickness 16 speculum blank would weigh 170 lbs. ________________________________________________________________________________ Do you knit? crochet? sew? make jewelry? do metal (or other) sculpting? build furniture? create with your hands and heart? If you do anything ‘crafty’ please consider doing one of our Sunday afternoon/evening diaries. You can volunteer by replying in the comments or by sending vgranucci a Kos-message [END] --- [1] Url: https://www.dailykos.com/stories/2026/8/2/800078894/community/waywo-stop-casting-porosity-2/ Published and (C) by Daily Kos Content appears here under this condition or license: Site content may be used for any purpose without permission unless otherwise specified. via Magical.Fish Gopher News Feeds: gopher://magical.fish/1/feeds/news/dailykos/