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Polishing

Polishing is a bit like grinding. Instead of carborundum grains scratching away little bits of glass, we use cerium oxide (CeO) on a pitch lap to smoothen the glass. The exact workings of this process are still unknown, but most explanations go towards the idea that the glass liquifies locally. Little holes in the glass are not any more scratched away like with grinding, but fill up with the liquit glass. Like I said I need a pitch lap for this and that lap has got to be made first ...

Wednesday 12 april: in Hamme I pass by coincidence a shop called Super 5000, and also by coincidence I find an electric 1600W cooking plate (746 fr).

Saturday 15 april: at Schleiper in Ghent I buy a bottle of line oil (75ml, purified, 109fr) and a bottle of turpentine (1 litre, 230 fr). The line oil and the turpentine are used along with bee wax to make pitch softer and less brittle.

Tuesday 18 april:8PM: I cut the pitch into pieces. The pitch comes in a kind of can that is open at the top. With a screwdriver as chitel and a hammer, I cut the pitch into little pieces; to avoid that the bits of pitch fly around everywhere, I put everything into a plastic bag. I'm busy with this for an hour and 15 minutes and it's been enough for today; let's try if these things want to melt also. Alas, there's no power cord long enough in here (I found out that of course *after* having installed all the other equipment). So I use the rest of the pitch to react off my feelings :-) After another half hour, everything is reduced to little pieces.

At the left the ceriumoxide, at the right the pitch
(Click picture to zoom in)

Thursday 20 april: I try to melt the pitch in a cola can of which I removed the top with a cutter. Alas, even after 15 minutes of heating, the pitch is only a little fluid (very sticky, like syrup). This is probably because the bottom of a cola can is not flat, so only a rather small part of it comes into contact with the surface of the cooking plate.

Saturday 22 april: At Hema I buy a small pan (0,75 liter, 14 cm diameter, 290 fr).

Wednesday April 26th: 9h20: I melt the pitch in the pan. Using some wood, I make a oblong rectangular shape and cover that with aluminium foil. The aluminium foil is painted with turpentine using a painters' brush, and then I pour the pitch in it. This takes about twenty minutes, then I remove the aluminium foil with the bar of pitch, which in the meantime has cooled down. Then I repeat the process, so now I've got two long bars of pitch.

9h45: All is done; I let the pitch cool down further. After fifteen minutes, the pitch is rather cool (I can touch the cooking plate and the pan without problems) and has become very viscose (or how do you say `sirup-like' ...).

Saturday April 29th: 21h15: I cut the two bars of pitch in square blocks. I use a cutter knife to do this, holding it into the flame of a lighter---when the knife is too cold, the pitch breaks in two (most of the time in a direction you don't want to break it in) ... The aluminium foil is also hard to remove. Maybe I used a little too much turpentine Wednesday on the aluminium foil; the pitch sticks too much to the foil, and mostly at the corners, which is the place where probably most of the turpentine went.

One thing is for sure: your hands will be covered with pitch ... The pieces of aluminium foil come off better using a hot knife. Also important is to remove the knife fast enough from the pitch, or else pitch and knife stick together! It appears that for each square, I have to re-heat the knife for a second or ten. Every two to four squares, the knife has to be wiped off on a scrap of aluminium foil, because the pitch that sticks to the knife has the tendency to burn by itself when I heat the knife.

22h25: I've made 24 squares. Now, I cover the tool disk with turpentine (again using the brush). This causes the pitch to stick well to the surface (just like we discovered with the aluminium foil, heh). I hold each square for five seconds into the flame and then press it onto the tool. The square itself I hold of course in my hand using a piece of aluminium foil, because I don't feel like roasting myself :-) At 23h10, the tool is covered with squares.

Tool just before and just after the pitch squares are applied

Monday May 1st: 10h30: I put a sheet of aluminium foil on the pitch lap, on top of that goes the mirror and the whole is pressed by a water bottle of 1.5 litres. After a few hours, not much difference is discernable. Na een paar uur is er nog niet erg veel verandering te zien.

Thursday May 4th: At 11 o'clock in the morning, I have another look and what do we see: the pitch lap has very well taken the shape of the mirror, but a lot of the pitch squares have come together (they've been pressed flat so that they expand a little sideways).
8.00--10.00PM: I re-cut the squares. I use a cutter knife that I put into the flame of a firelighter. This method will later on turn out to be rather clumsy. One of the squares almost completely breaks off during the cutting; I manage to fix it by putting it back on the glass disk and holding it into the flame of the lighter, so it melts to the surface.
At 10PM, I let everything push again under the 1.5-litre bottle.

Friday May 5th 6.00PM: bottle is removed.
8.30--9.00: Re-cut the squares.
9.00--: bottle put on again.

Saturday May 6th --11.30AM: bottle removed. The squares have again been pushed against each other; it seems that I've been rather enthousiastic and such long pressing times are not really necessary.
11.30--12.15: recutting squares; now I use another method: put the cutter on the square at the place where it needs recutting, and give one (or two or three) ticks with a hammer. The cutter works in other words like a kind of chisel.
1.25--2.10: disks are pressed (as always by putting the water bottle on top of the disks and letting gravity do its work).
5.35--7.35: polishing, MoT 1/3W. My method: put a bottle of water on top of the mirror disk to apply pressure; the rest is more or less the same as with grinding (the 1/3 W-stroke for example is exactle the same).

The tool, fastened to the plank with four screws. The plank itself is clamped to the table using two clamps. Every five minutes, I turn the mirror disk about 45 degrees, and every 10 to 15 minutes, I turh the plank (with the pitch lap on it) 90 degrees in the same direction. The first wet lasts about 55 minutes. After that wet, the squares have become much flatter (the ripples that could be seen before are almost vanished). At 7.35, I quit (this is a rather tiring job) and after washing and drying the result is already visible: the whole surface of the mirror is reflective now!

8.30--9.00: A few of the channels are almost filled up with pitch because again some squares have been pressed flat during the polishing, so I recut everything.

Tuesday May 9th: 7.25--7.35PM: recut squares.
7.35-8.00: pressing (now with a two-litre bottle of water).
8.00--10.30: Polishing, MoT, 1/3 W-stroke. I use a mixture of 7 teaspoons of water and 1 teaspoon of powder. The first wet lasts about one hour; at the end the motion is so difficult that a few bits of pitch jump off; a bit of it sticks on the mirror! Washing this under the faucet helps a bit, but not all the pitch goes away. Happily ehough, the problem solved itself when I continued with fresh water and powder.
Afterwards another half hour recutting the squares.

Friday May 12th 9.00--9.45PM: pressing
9.45--11.50: polishing (first wet lasts 1h5min). MoT, 1/3 W.
At the end, I clean the mirror with water, but a little fuzzy layer of powder sticks to the surface, even after opening the faucet fully. The solution is simple: before you stop polishing, do a few last strokes with much water and almost no powder.

Saturday May 13th 1.25--1.50: Recut squares.
1.50--4.00: pressing (bottle of 1.5l together with the bottle of 2l)
5.20--6.10: idem
6.20--7.55: Polishing, MoT, 1/3W. The first ten minutes, everything goes rather unfluid: the mirror shocks a little during the pushing and pulling. The first wet lasts 1h10min.

Saturday November 5th The National Star Watch Days arrive again! Our friends of the VSRUG show a.o. their Foucault-tester, an occasion that we should not let pass of course! These experienced people have a look at the mirror and: only at the middle, the mirror is a little bit hyperbolic (solution: 15 to 20 minutes of polishing with a long 1/2 or 1/3 W-stroke)! All the rest is okay (no turn-down edge, no remains of pits on the mirror---so I've polished long enough---, no scratches)!