Wow… Fifteen month’s after I first laid hands on my glass, I have finally begun polishing. Let me say up front that so far no part of the process has been anything like I imagined. I’ve read the literature, I’ve read personal accounts like this one, and none of them conveyed the sheer diabolical nature of a brand new pitch lap at the beginning of polishing! The coefficient of friction was a meaningless term, as it switched from “superglue” to “Greased ice”, sometimes several times in the course of a single stroke. Happily, this didn’t last long, and things settled quite rapidly, although I think it will continue settling down for some time yet.
I arrived at the class, packing the mirror and tool in separate containers, and with a nice new polishing board. The board was made from an old square of supawoodTM with retaining pegs made from stacks of rubber washers fixed into place with screws. To keep it waterproof, I gave it several coats of eye-scorchingly bright green enamel paint which we had lying around from an earlier project. This turned out to be the first mistake, as I hadn’t allowed enough time for the paint to cure – although it was dry to the touch, it remained slightly tacky, and was still wet beneath the surface. Any significant pressure or friction caused it to break free from the undercoat, and spread a thin layer of wet paint. The result is that there are now pretty little spiral ridges of recently dried paint directly underneath the tool. I’ll have to fix this.
The thing is, after 15 months of grinding, I was pretty comfortable with the routine. I would set up and get to work quite quickly, I even helped some of the newbies out without worrying about the quality of my advice. But polishing was entirely new territory, so I ended up standing around hoping somebody would volunteer to help me. And they did! One of the younger members of the class, Montague, helped get me started with pouring a lap, although Johan stepped in after a while to offer the benefit of his years of experience. The process was simple: Clean and dry both the mirror and the tool. Wipe a few drops of motor oil on the surface of the tool, leaving the thinnest possible film, as a wetting agent to help the pitch to stick more firmly. Place the mirror face-up, with an appropriately sized mould resting on top, so that it conforms to the shape of the mirror. Melt some pitch (previously confirmed to be of a suitable hardness), and fill the mould. Place the tool face-down on top of the pitch and gently hold it in place while the pitch sets. As the pitch becomes brittle, break away the sections of pitch overflow (when working with plastic moulds, this excess material is called ‘flash’. Does that word also apply here? Answers on a postcard to firstname.lastname@example.org), until the pitch has set enough to become brittle. Peel off the mould, take a putty scraper and chip away the rest of the excess pitch (anything protruding past the diameter of the tool). And then the lap is created and ready for pressing!
This was when I realised I’d forgotten to find a bottle to hold the polishing compound and had to use the class bottle. A few splashes of the compound (actually a thin slurry of cerium oxide powder suspended in water. It’s an interesting colour – I remember when I was a child, my crayons always included one labelled ‘Flesh’, which was about the same colour. Incidentally, I never met anybody of any race whose skin looked like that crayon…) onto the mirror later, place the new lap face-down into the mirror, and apply pressure. For the first few presses, I simply leaned on the glass with my full weight. This puts a lot of pressure on your arms, so I soon switched to just balancing a bucket of water on top of the stack and letting it stand for a break. Pressing is important because it presses the pitch into increasingly intimate contact with the mirror surface, until it conforms exactly to the shape. Once this is done, we say that it has good contact, and we know that all parts of the surface will be polished evenly. This process also embeds some of the cerium into the surface of the pitch, which is critical to the polishing process.
Once that first pressing was complete, I could begin the actual work of polishing – with the mirror still on top, I pushed the tool back and forth in a wide W stroke with one third overhang – exactly the same as the early grinding stages. And this is when I experienced the practically uncontrollable conditions I described at the beginning of this update. After about 15 minutes, I put down a fresh charge of cerium oxide slurry, pressed some more, ground some more. After the third pressing, I turned everything over and polished mirror on top – I will continue working in this way until polishing is complete and I have achieved a perfect spherical shape.
Once that is done, I begin the final stage: Figuring. I’m given to understand that this is a form of sorcery, so I won’t spare any thought on it until I absolutely have to.
A final point: The reason a mould is used when pouring the pitch is simple: a lap must be channeled, so that it looks like an array of square facets (although I’m sure the actual shape is unimportant. Squares are just easier to make). Traditionally, they would be pressed into the soft pitch, or cut out with a knife. This sounds like a terribly messy and fiddly process. We used soft re-usable moulds instead. No mess, no fuss, just a perfectly regular set of well-spaced squares.
So why are the channels necessary? Two reasons: First, pitch is an extremely viscous liquid. You know the popular myth that glass is actually liquid and flows slowly over the centuries? Well pitch really is like that, which is why it’s used to polish optical surfaces. It’s rigid enough to polish the surface, while continuously conforming to the surface of the glass as it gets polished to new shapes. But if it is to flow, it needs somewhere to go, and this is provided by the channels. The second reason for the channels is that most of the polishing work is done at the edges of each facet. Without channels, the only edges would be at the outer perimeter of the lap, causing it to polish around the edge of the mirror while barely touching the centre.
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