WE receive a steady stream of inquiries from amateurs who want to coat their own mirrors by the evaporation process, and our usual reply is: If you have previously dealt with high vacuums, or if you are prepared to do some months of preliminary wrestling with the process, also if you have something under 100 dollars to spend, go to it-you will have a lot of fun or at least "experience." But don't run away with the idea that you can do the job as simply as some seem to believe, or anything like as simply, as the ordinary silvering job. On the other hand, the technique does not seem to be a big obstacle to a man who has had the advantage of a general physical laboratory background. Here is a letter from Gerald E. Kron, 405 La Follet, Adams Hall, Madison, Wisconsin, whose connection with the University of Wisconsin provided him with that advantage.

"Your letter gives me the impression that the amateur telescopists consider evaporating metals quite a difficult piece of work. In a sense this is true, but if one has all of the equipment available, as I did, the evaporation of silver, at least, is quite simple. I built the evaporating outfit and got a coat of silver all in the space of two weeks. But it must be held in mind that I had all of the means of a large university at my disposal, in addition to the advice of several people who had done evaporating before.

"The most important condition for evaporation of metals is to have a high enough vacuum. I believe that any trouble experienced by people who have failed was caused by having too poor a vacuum. It is necessary to have a pressure at least as low as 10^-5 mm. of mercury, and a very good coat will be obtained if the pressure can be reduced to 10^-6 mm. [About 1/75,000,000 atmospheric pressure.-Ed.] Aluminum is very difficult to evaporate at any pressure higher than the second named above, because it tends to burn up, or acts as a 'getter.' Silver is by far the easiest of all the materials that I tried, and I would advise anybody experimenting with the process to start out with it, by all means.

"I produced my vacuum by first pumping down as far as I could with a Cenco [Central Scientific Co. 456 E. Ohio St. Chicago.-Ed.] Hi-vac fore-pump (about 10^-3 mm. of mercury). Then the final high vacuum was produced by surrounding a charcoal trap fastened to the apparatus with a dewar flask full of liquid air. The amount of air and other contaminating material that such trap will absorb (or adsorb) is almost unbelievable. It will reduce the pressure from fore-pump pressure to such a low point that an electrical discharge will not pass through. I used this as a test of my vacuum. It is crude, but quite convenient, and after one learns something about the character of discharges through different pressures he can estimate his pressure with a fair degree of accuracy.

"By far the greatest difficulty encountered was to eliminate all leaks from the system. I believe that the only person in the world who really learns something about leaks is the man working with high vacuum. I know that any of the men working at Wisconsin University will agree with me, and there is a great deal of high vacuum work going on there. The character of the apparatus made it very susceptible to leaks. The apparatus being composed of a large bell jar over a very heavy vacuum plate in which were sealed the necessary electrodes and tubes. Bell jars for such purposes are usually sealed down with a wax of some sort but I used common Plasticine, a type of modeling clay which never hardens. It makes an excellent seal which is very easy to apply and remove. [ J. L. Hammet Co., Cambridge, Mass.-Ed.]

"The material to be evaporated is melted with a heavy tungsten filament supported by large electrodes sealed into the vacuum plate. It is difficult to find a material with which to seal these electrodes against the vacuum, and still have them insulated from the metal plate. One of the research men made a glyptol resin for me which is just about the acme of perfection for the above purpose. If anyone you know of would like some of this stuff, I shall be glad to tell him how to make it. It can be melted in place, and yet it is proof against the intense radiation from the filament.

"Many materials can be deposited by evaporation: silver, copper, aluminum, gold. platinum, chromium, quartz, and fluorite. I tried chromium and quartz, but I was not able to get them to a high enough temperature. I have found out, since, that chromium does not give a very bright surface, anyway. Quartz is useful, because it can be deposited over a surface like silver, giving it almost perfect protection. As a whole, aluminum does not seem to stick as well as silver. The latter must be removed from the inside of the bell jar after a run, with an acid or an abrasive powder.

"If anyone you know of would like some help with this evaporating. I shall be glad to answer any questions that I am able to."

The advantage of aluminum over silver is, of course, that it automatically coats itself with oxide which, being corundum, has the hardness of sapphire. Still, anyone who intends to play with the evaporation game probably would pick up valuable experience from the experiments with plain silver suggested above, later trying aluminum.

A LOT of people are having trouble with pitch for laps. Joseph A. McCarroll, an architect, 521 Palisade Ave., Teaneck, N. J., had so much of it that he got mad and made an investigation of pitch from all angles and, as a result, has decided that the best pitch is not the pine pitch most of us use but coal tar pitch. Only after he had done a lot of experimenting with coal tar pitch did it become known that Ellison now uses that kind. Here is what McCarroll writes:

"Many mirror makers may think that, because there seems to be little or no reference in print to the use of coal tar pitch as a base for polishing, this material is therefore not suitable for that purpose. It will come as a surprise to many to learn that it is not only suitable, but that in many respects it offers some definite advantages.

"The material to use is coal tar pitch. It is a by-product of the manufacture of illuminating gas. The tar is usually sold by the gas companies to concerns that distill it to obtain its many very valuable derivatives so useful in the arts and commerce. Pitch is what is left of the tar after distillation. Coal tar pitch should be distinguished from 'water-gas' tar pitch, which is not nearly so useful to the mirror maker.

"Among the advantages of coal tar pitch is a rubbery smoothness of the surface texture of the material when formed in the lap. This probably accounts for the excellent polishing effects obtainable. It seems to work faster and with more positive effect than pine pitch, especially in figuring. It seems that the tendency to cause scratches is less. This may be due to the absolute purity of the material. It is said by the makers that the ordinary run of the pitch, as manufactured, is entirely free from grit or any other foreign substance. It is not necessary to strain it, provided it is packed in clean containers.

"In melting coal tar pitch it is advisable to watch it constantly, especially where the melting point is high. There seems to be a point during the heating when it may suddenly froth up and run over the container and become ignited by the flame. The danger from fire should be carefully guarded against. Water-gas pitch is particularly bad in this respect and should not in any circumstances be used for tempering purposes.

"In making the polishing lap the technique will be found to be quite similar to that used for pine pitch. There will be a marked difference, however, when it comes to cutting facets, as coal tar pitch is much less subject to chipping under the knife. This is a tremendous relief and this one quality alone should commend it highly.

"Such materials as wax, turpentine and rosin cannot be used to advantage for mixing with coal tar pitch for modifying its hardness or other properties, for this kind of pitch is entirely different in chemical composition from pine pitch and, in fact, in making mixtures of either of these materials only such as are of an allied nature should be used. It is true, the two differing varieties of pitch can be mixed, after a fashion, but the smoothness of texture of the coal tar pitch is then lost and gas bubbles will be found to be one of the troubles. Instead, pitch having a higher melting point should be used to harden a similar pitch of low melting point, or vice versa. Creosote oil (common variety) may be used for softening the pitch, although it is better to mix pitch with pitch; if for no other reason, so that one can exactly determine the melting points of the components. For example, equal quantities having melting points at 150 degrees and 200 degrees Fahrenheit will yield a mixture having a melting point of 175 degrees. The range of grades which should be kept on hand for all year round use should run in about 10 degrees intervals from M.P. 140 to 180 degrees. The extremes are useful chiefly for mixing purposes since, if used straight, they would be too soft and too hard, respectively. A melting point of between 155 and 160 degrees will be found to be most generally useful for polishing at room temperatures around 70 degrees. The determination of the melting point of pitch is accomplished by the use of very special testing devices which the amateur would not likely want to bother about. He should get his pitch already graded for melting point.

"The melting point is a very definite property of the pitch, which must be known for basic tempering purposes, but for determining the probable behavior of pitch on the lap under any given room temperature it is necessary to have some other test, such as the penetration test, which will show the specific hardness under existing conditions. This distinction will be apparent when it is realized that a pitch with a melting point of, say, 150 degrees, used in a room at quite low temperature, might have exactly the same specific hardness as a pitch of 170 degrees M.P. in a room of higher temperature.

"Unfortunately the writer, at the present moment, is unable to give very definite suggestions regarding available sources of supply of coal tar pitch in small quantities. It is commonly sold by the ton and in drums containing several hundred pounds. Almost any local roofing concern will have a supply of it, but when using material from such sources straining will be necessary and considerable experimenting with mixing and tempering will undoubtedly be required.

"The writer has been using coal tar pitch for quite a time, under the impression that he was, perhaps, something of a pioneer. Word has recently come, however, that Ellison is using it and favors it for figuring. This should definitely put the stamp of approval on its use."

We intended to reserve this paper for inclusion in the "A.T.M. Supplement." but thought it perhaps better to publish it here first, hoping that other experimenters will report their findings on this kind of pitch before it is set up permanently in that book. In the meantime a source of supply may turn up.

Until we receive a reply from Ellison, all that we can say about his use of coal tar pitch is contained in the following sentence from his last letter. "Coal tar pitch-the very stuff that the paviors use for setting the street blocks-is the best of all for figuring." We have a sample of the pitch he was apparently using in 1928, when we visited his shop, and this appears to be pine pitch; hence he must have taken to using coal tar pitch since that date. Just as we read proof on these pages a rumor that Fecker use it turns up. Mr. McCarroll at the same time (latest minute) believes he has located a source of supply from which the prepared product may be obtained, but this is as yet too indefinite to state here. Although he is busy, we have urged him to be the dealer, himself, because he understands the requirements from the point of view of the amateur telescope maker. Whether he will consent or not, we cannot say, but may have more to say later.

SOME who have written to individuals mentioned in this department, also in the instruction book "Amateur Telescope Making," have evidently ascribed their delayed replies to unwillingness to answer. It is seldom or never that. It will be evident that, when a man's name and address is given here, a flock of letters will converge' on him from all directions, and if only on out of a dozen of these includes a stamped return envelope, that one fellow stands to be out quite a bit for postage alone, and his delay may be understandable. Also make it easy for him-tell him to scribble his answer on the margins and back of you letter; why be conventional among fellow hobbyists?

RECENTLY, when writing to Wallie Everest, your old scribe included sketch of his wonderful, patented, standardized cold pressing equipment, and Wallie returned it with a sketch exhibiting various scientific methods employed in Pittsfield. Both sketches are reproduced on this page. In our own sketch, note tree- trunk pedestal; also lever across top, one end bearing on a shelf and shiftable endwise in order to get variation in pressure when single hanging weight is used, and the other end bearing on the handle dingbat shown in A.T.M., page 288, through pivot point A, an alley stolen from Scribe, Jr. Two nine pound hemispheres of lead and an 18-pound chair may be added for heavy pressing, also "Skow," the cat, though she won't often stay put in the chair. (Contrary critters-cats.) Like Topsy and A.T.M., this remarkable outfit "just grew,' but we will bet it is no worse than some which the rest of you readers are using and not so bad as some-and it works. When using heavy weights, rapid unloading may cause deformation of the mirror, hence inventor Everest suggests a washtub which may, if preferred, be filled with champagne and this weight siphoned off, at the end into the sink-or elsewhere.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.