Quartz Crystal Memories – Eric Vast

During the three of four years I ran a quartz crystal factory, we found a number of interesting things which some people may not have met and as far as I know are still not fully understood. After we had learnt the hard way, we attained an output of over 1000 finished crystals a month and operated a service of any frequency. Two crystal in half an hour so we had plenty of experience. During this article I shall try to stick to the word “crystal” as the finished product we all know, “plates” as the raw quartz being processed and “quartz crystals “ as the raw quartz as found in caves or dredged up in river gravel in its natural hexagonal crystal shape.

ACTIVITY: Apart from the coupling of modes as carried out by edge grinding, which I will deal with later, the shape of a plate has a considerable effect on activity. When hand finishing the quartz plate is gently rubbed on a piece of plate glass loaded with spent grinding paste by a finger in the middle of the plate. This tends to produce a slightly concave plate which is quite dead. At first we tried to get the plates exactly parallel. We had means of measuring the contours of the plate optimeters which measured up to 1/250,000 th. of an inch and by trial and error we found that plates which were slightly convex like a lens had increased activity. Accordingly this became the standard shape. After the war I wondered if this was worth patenting but I found the Dutch conglomerate of Philips had already done so a few years before. Their patent gave no hint of why the technique is successful with shear mode plates. I have never seen any references elsewhere let alone any explanation of the effect.

BATCH PRODUCTION: We had an interesting device for the production of a batch of identical frequency crystals. You take two similar cast iron annular rings about 15” across and a rectangular section about 3 by 2” with one on top of the other. You make an insulated carrier of Bakelite or some such material, and a complete disc to go between. Where it is between the rings you cut 5-sided holes rather larger than the plates to be worked. The device will “tumble” the plates and will slowly rotate them, if thinner than the plates and if rotated eccentrically by a central hole in an off centre crank. Holes to feed in grinding paste are made in the top ring. If the grinding paste is made up with an insulating oil, and not with the customary water, the small random electric charges caused by the grinding are fed to the two rings which are kept insulated from each other. With a dozen or so plates a signal of around one half a microvolt is obtained. This we could measure and listen to on a good HRO and it sounds almost exactly as the noise made by the operation when you listen to it by ear. At first with fresh cut plates the noise band is wide, say 50 kHz, but as the process continues it narrows to around 10 kHz. If the plates are removed at the right time they are near the desired frequency and very much alive. They can be hand finished of course. It is interesting to follow the slow shift of frequency towards the target and the narrowing of the band width as the work progresses.

CRYSTAL HOLDERS: Getting holders was the bane of my life. The plates in those days were about I.” square and electrodes were all shapes and sizes. However lightly clamped a shear mode plate is. in the final assembly, it most touch the electrodes somewhere and be in contact with the two clamping electrodes and there must be some loss of activity due to this. In the end we settled for a square plate of metal with the centre milled down, about one thousandth of an inch, leaving four tiny corners as supports. We found that the material of the metal electrodes was very important. Most people seem to use stainless steel, but we found certain grades of nickel alloys ( German Silver ) had far less friction. After prolonged use the crystals would produce marks on the plates from the metal at the corners holding them. Under the microscope this marking could be seen to be a smear of metal, so evidently the plates were actually moving where clamped. The Bakelite cases we got from Palestine where there were plenty of would be moulders. The quality of some of the mixtures was such that they sweated oil and queer chemicals for years. This of course got onto the plates and they lost activity. However in the end we found one character who had some good Bakelite. It was suspected that he bought it from the Germans via Turkey, but we weren’t particular, it worked.

ETCHING: Quartz crystals, either natural or artificial, come in two kinds. Left-hand and right-hand. When these are fairly complete they can be identified by certain of the facets. It is necessary to know the “handedness” of the quartz for setting up when cutting otherwise you will end up with the wrong angle of cut. Natural quartz contains both “hands”. Artificial quartz is usually pure but expensive and difficult to grow large crystals. Natural quartz is formed underground, in caves and crevices ( Known as Vugs, usually pronounced voogs. Ed. ) where volcanic gases have heated the place up. Water under high pressure and temperature can dissolve silica. This needs to be cooled slowly over the years to produce quality crystals. The inclusion of both “hands” in apiece of quartz is known as twining. When cutting crystals one tries to avoid the unwanted parts. There are, according to the pundits, three sorts of twinning, ie Electrical, Optical and Combined Electrical and Optical. I have always got on well by assuming that I am most likely to encounter the Combined version. In fact, I don’t think we ever identified the first two types at any time. Combine Electrical and Optical twining can be spotted in polarised light by looking through the clear crystal faces. Plates off the cutoff wheel do not have clear surfaces and quartz is not easy to polish. I had a gent who had polished glass for years and even he found it very difficult. At the polishing pressure required quartz breaks off corners and edges in flakes. This leaves etching. When quartz is immersed in hydroflouric acid it dissolves. If you take a sphere of quartz ( not at all easy to do ) and suspend it in the acid it partially dissolves and leaves a cuirious regular shape which is not at all like that of the original sphere (It is fact a reflection of the underlying molecular structure which defines the natural shape of the crystal. Ed.) This means that the rate of solution is different in various directions. Now when a surface is etched in the acid bath it suffers a pitting of the surface which varies in slope according to whether the quartz is right or left handed. If such a plate is illuminated the pits, according to their angle, give the effect of different areas of brightness. They can then be marked and avoided in further cutting. The mystery of this is why the plates pit at all. No gases are emitted during the reaction so bubbles are not responsible, nor apparently is there any question of microscopic defects in the quartz. Learned papers have been written on this subject but I have found none particularly believable.

One of my chaps who went on to better things invented an X cut ring which was independent of twinning. This greatly delighted H.M. Post Office of those days, who had some five tons of badly twined quartz in their cellars! Such rings (a sort of folded X cut bar if you think about it) are used in Carrier Filters. Etch pits must be a gift of the gods.

X RAY ADJUSTMENT: Just before D Day I was in the UK having a good look round and acquiring bits and pieces for the factory. Some of the crystal units of the big boys like Marconi had a curious device acquired from Philips< Canada, and said to be in use in the States and Canada. Basically it consisted of a big X Ray tube set up to irradiate a dozen or so crystals at a time and was said to be capable of adjusting its frequency over a wide range. A measuring device was inside it somewhere. These things were very professionally built and apparently a hundred or so had been produced.

Enquiries met with a blank wall. “Yes, it worked OK.” Did the frequency go up or down ? “ Well the chap who works it isn’t about today and I don’t really know much about it.” So when I returned to base, I took a selection of plates up to the local hospital and got them irradiated in various degrees. Some indeed to knock oxygen molecules out of position and leave black spots of silicon. The result was curious -very little change in frequency, some up some down. I Later I heard that the whole project was said to have been based on a sample of quartz which contained considerable amounts of impurities. Rather a waste of time and money I

VACUUM MOUNTING: Before I was demobbed I did several months at SRDE in the Crystal Section. At that time they had started to mount various types of plate in a vacuum and usually they were sputtered with a gold surface. Being a largely – civilian establishment, many were the intrigues and politics. The Crystal Department was run by a lady physicist nearing retirement, and many wanted her job. At one high powered meeting she had been attacked for gettering ( using a film of silver ) to get a high vacuum. She asked me to investigate. So for a start I took a mounted plate of some 6 – 7 MHz and connected it to our best pump. I also arranged a slow leak which could be switched on and which would let most of the air back into the valve sized envelope in some 15 minutes. I connected the electrics to an activity meter. It had never occurred to me that acoustic waves in air could happen at frequencies of MHz. If it had as the wavelengths are in thousandths of an inch I would have expected any resonance effects in a space measured in inches would have been minimal. The speed of sound in air, wavelengths etc do not vary much with pressure. How wrong I was. I got the vacuum down to about 0.01 MM and then turned on the leak. The activity went all over the place from the word go. I suppose about 500 or so peaks during the 15 minutes I gave it, fairly irregular. – obviously standing sound waves. Between peaks there were as many zeros. It seems incredible that the sound we listen to is capable of producing such high frequencies. It would be nice to know how much higher it can go. Bats work at 80 – 100 kHz but this was much higher. We kept to a very good vacuum after that.

BREAKDOWN: Sometimes we would find that some piece of transmitter equipment was eating crystals. This inevitably was due to high RF current in the crystal. We organised some tests and found rather what the complaints had indicated viz. low frequency crystal, say around 2 MHz, were puncturing at upwards of 2 mA of RF but that at 9 MHz they seemed to need 8 mA or so for failure. This of course gave us the data to reduce the failure rate but still left the question as to why. A low frequency quartz plate can be considered as a pile of higher frequency ones. Hence I would have thought it should have taken without damage what the component parts could still stand, perhaps even more. Does any body know why ? If so I would like to hear from them.

EDGE GRINDING: Edge grinding has existed as long ar there have been crystals, but was not properly understood or investigated until the late twenties when it was studied by the Bell Laboratories. It had been found experimentally that a finished plate could have various degrees of activity as prepared with various widths, but if either edge of one particular pair of edges was ground away, the crystal could ultimately be greatly improved in activity. The other pair of edges had little or no effect if operated on. The Bell Labs work indicated that the greatest activity of the crystal was obtained when the distances Involved were such that an even harmonic of the flexure mode of The plate equaled the shear mode ( i.e. 1st shear mode ) of the  plate. Such even harmonics are of the order of 2,4,6 for low frequency AT cuts and much higher, say upwards of 12, for higher frequency BT cuts. The formulae involved are quite simple. The width of the active section is noted and a harmonic calculated which can be reached by removing the minimum of edge. The process works like a charm, and is quite routine. The rules having been established, the reasons were sought, but to the best of my knowledge the explanations have never fitted the facts. Early suggestions were that the plate was in fact moving in shear mode and in flexure at the same time. However what we know of the action of mounted plates indicates that the motion is truly shear and any additional flexure must be very small. The best suggestion is that the attempted coupling occurs at the edges only, and that inside the crystal there may be only relevant stressed areas. If you draw out a shear mode and a flexure mode as above the two edges both move in a similar fashion which is a least a start.

THE QUARTZ CRYSTAL AS AN OBJECT: It is very curious that the raw quartz crystal is reasonably consistent inside, but very variable externally. Normally the ones worked are really halt crystals. This is how they grow on a rock wall. A complete crystal is double ended and has 30 major facets. These have all been coded by crystallographers with letters. The ideal shape of a facet or even the whole crystal is a very rare occurrence. By luck the angle of the facet face to the main axis doesn’t vary. This greatly aids the setting up for cutting. The inside of the crystal is a very complex structure. When .it’s physical properties are measured e.g. elasticity. Young’s Modulus, optical refraction, piezo electric properties, conduction of heat etc. etc. the pundits have found it necessary to define in some cases as many as six constants associated with the axis of the crystal. If you would seek more information on this there is a book  “Crystal “ by one Mr.Wooster, not Bertie, which can be extracted with difficulty from most County Libraries. In past ages the ones we use i.e. the clear ones, were known as Rock Crystal. It is a most difficult substance to work into an artistic shape due to its hardness and a tendency to flake. Often very tiny drops of water are found inside. Quartz coloured by impurities has attained the position of a semi precious stone e.g. Amethyst.

Whilst we were operating, a search for quartz was made in the Eastern Desert. It yielded several tons including some enormous crystalline lumps, two feet long of a very pleasant pink colour. All very badly twinned. Also one carved lump, of Quartzite, carved by an ingenious Arab into the shape of a quartz crystal ! The largest quartz crystal ever found was 2 metres long and 1 metre across each face. The Japanese at one of their exhibitions many years ago showed a 1 metre diameter polished sphere of reasonably transparent quartz. I imagine it is still on show somewhere.

Nature managed it apparently. Why not in your spare time try and grow an even bigger one ? You only need silver sand and water, about 300 degrees C and 500 atmospheres, and of course at least a year cooling.

Eric Vast – October & November 1989

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