In this chapter we discus the polishing operation by means of polishing pitch. A few years ago we only used the pitch lap to polish the mirror. The great disadvantage of this method was that it took about 4 to 6 hours to completely polish a 150 mm mirror. But with the introduction of the method with the felts, we obtained a serious gain of time. In the chapter about the felts, you can read about the many advantages of this method.
For those having problems obtaining felts, the method of polishing the mirror with a pitch lap will be the only alternative. They have to follow the procedure described in this chapter entirely.
If you polished your mirror entirely with felts, you still have to finish it with the pitch lap to obtain a perfect surface and figure.
Polishing with the pitch lap
If you ground your mirror with carborundum no 1200, the surface isn't already finished. To work out the mirror we have to use an other method. The combination of glass and carborundum doesn't give us the surface characteristics we want to obtain. There are even finer carborundum grains (no 2400 and no 3600), but practical experience tells us that the risk for scratches increases and it don't bring gain in time. Therefore we have to look for an other way to ground the mirror (from now on we call it polishing). A good way to do this, is by using pitch in combination with ceriumoxide or 'rouge' to obtain an optical surface. Most grind packages contain some pitch (500 gram will suffice for a 150 mm mirror).
The first thing we have to do is to break the pitch in small pieces. For that we put the pitch in two very clean and strong plastic bags and beat it with a hammer. Afterwards we put the pitch in a cooking pot or a tin can and melt it. The pitch will melt in less than 10 minutes. Take care that the pitch don't warm up too much. If the pitch boils, it will become too hard and be useless for us. We carefully mix up the pitch and pour it over a mould. This moult is a made of silicon and it's shape is like a waffle. The pitch lap have to be about 1 cm thick. Now we wait a little bit till this pitch lap is solidified.
In the meantime we make a creamy mix of ceriumoxide and water. We put the mix in a little film container and shake it. We then rub in the tool with white spirit. If the pitch lap has cooled sufficiently, we can put it above the convex tool. Now we paint the surface of the mirror and the pitch squares with the creamy mix of ceriumoxide and water and we cold press the lap between the tool and the mirror. We press the mirror with weight to be sure there is a good contact between the pitch lap and the mirror. This can take you half an hour or so if the pitch lap isn't warm enough. When you do this, be certain you turn around the mirror every now and then to avoid that the mirror and the lap don't stick together. If you can see through the upper side of the mirror, then you can check if the pitch squares do match the mirror surface. Keep pressing the mirror until all squares shows good contact.
If you notice after a while that the facets doesn't change any more despite the mirror and tool aren't yet fully in contact, then the pitch has become too cold and he must be warmed up again. You can do this with the aid of an infrared lamp. But don't warm up the pith too much because it may chill and adhere to the mirror. Before we press the mirror again, be sure to paint the mirror and the pitch squares with the mix of ceriumoxide and water. If it takes to long to obtain full contact, you can consider to put a weight of 10 kg on top of the mirror for 30 minutes. You may even sit on top of the mirror for a while but take care to sit above the centre of the mirror, otherwise the facets at the edge will deform. Now it is important to check if the square edges don't touch each other.
If that happens, you have to cut the edges vertically with a sharp knife. Be sure the squares all have the same shape, the same dimensions and are evenly spaced. The shapes may be round, square or even hexagonal but they all must have the same dimensions. We normally use squares, because a wooden mould with rectangular facets is easier to make than one with round facets and a pitch with rectangular facets is easier to correct. For the correction of the cold pressed lap we use a sharp Stanley knife. We have to cut very carefully. Use a very sharp razor and keep the razor angled away from the facets so they don't chip (half cutting, half scraping the facets). This needs some practice, but after a while, you will succeed in doing this quite well. Now you can put the mirror on top of the pitch lap, cold press a minute and check if all facets show good contact. If so, you can start polishing the mirror.
The basic polishing stroke is the same like the basic grinding stroke and consists of a normal one-third diameter. Start polishing slowly and gently with the mirror on the top, and always use the one-third diameter stroke. Don't forget to turn your tool and mirror like you did during grinding. Keep pressing the mirror all the time, watch the temperature off the room and put on some ceriumoxide mix every now and then. If the mirror suddenly hangs, then cold press the mirror for a few minutes and polish again. Maintain a steady stroke as long as and gentle you can. After ending one stroke gently start another one, otherwise the risk to obtain a turned down edge increases. If this error becomes too big, you have to restart grinding with carborundum no 1200. Already after the first ten minutes of polishing, the mirror brightens.
You might keep on polishing for an hour before inspecting the mirror surface with the little microscope for the first time during polishing. Now you might inspect the surface every hour so you can see the progress of your work. But don't forget to cold press the mirror every now and then and eventually warm up the pitch lap with an infrared lamp to maintain good contact. This is essential.
If the squares flatten too much and threatening to close together, then it is time to trim the facets. At the beginning, if the mirror is above, the centre is polished first. After a couple of hours, only the edge of the mirror shows tiny abrasive pits, these are the remainders of the carborundum grains no 1200. But don't give up the work yet. It is very important for a good mirror that every little pit is gone. But it cost a lot of trouble to remove the last pits, especially those at the edge. It isn't possible to polish your mirror entirely in one evening. If you stop, put the ceriumoxide mix on the pitch, lay the mirror on top of it and put both of them in a plastic bag to avoid that the pitch dry out. If the pitch dry out nevertheless, it is possible that the mirror sticks to the pitch lap. The only way to separate them is as follows : fasten the lap and mirror on the workbench and tap on them with a wooden hammer.
During polishing you sense a very uniform adhesion between the lap and the mirror. This should remain during the hole polishing phase. If you feel that the mirror drags with a heavy pull, then you are sure that the polishing is effective. If you suddenly notice that the mirror catches and glides while you polish, then the pitch has become too dry or the polishing mix contains too little water. It is time to put some ceriumoxide mix on the pitch. If that doesn't help, there could be another reason. Perhaps there is no good contact between the lap and the mirror. So cold press the mirror a couple of minutes. Or it could be that the pitch has become too hard. Pitch becomes too hard if too much dissolvable substances are evaporated. Therefore if you start to polish, be sure you can finish the work in two weeks at last. Otherwise you have to remove the old pitch lap and pour out a new one (quite expensive).
It isn't very easy to remove the old pitch lap from the tool. The only way to do this, is by hacking with a sharp object without damage the tool. Some people put the tool with the pitch lap in the
freezer so the lap come loose easier. You could cook the old pitch again, add some turpentine or beeswax to fill up the lost dissolvable substances. If you put in too much turpentine or beeswax, you can cook the pitch somewhat longer so it becomes harder. But we always recommend to use a whole new pitch lap and throw away the old one.
It isn't easy to explain how to prepare the mixture for the pitch. To make a good pitch lap, you have to follow your feelings. But don't worry, after a while you get 'it' too.
Pek en glas