Recently I purchased a couple of T3's on the cheap side and when I got them I quickly discovered they had problems. While they were fairly complete, it didn't take long to discover there were problems with the horizontal circle. I figured it was mold because that is something I had been told to be careful about. But as you'll see in a minute I really don't think mold is the problem.
At any rate, the first thing to do is take apart the lower end of the instrument. Fortunately I have a manual that gives me some step-by-step instructions - but not really all that concise. So I get it apart and start looking at the horizontal circle and here are 2 pictures of what I see. The first picture is of a part of the circle that is still in pretty good shape and if you look close you can even see the engraving. These are hard to photograph because of the reflective surfaces.
And here is a picture of the opposite side of the circle that has problems. I don't really think it is mold, but rather the interaction of the silver plate with moisture - sort of like what happens to a mirror around its edges in a bathroom.
So, how did I bring it back to life? Several years ago I had purchsed a T3 that I knew was missing parts figuring that someday I'd see about finding parts and putting it back together. Well, I wondered what the Horizontal circle looked like so I took it apart as well and it looked good. My first hope was to swap out the lower half of the instruments, but that did not work - parts are not 100% interchangeable. But I was able to swap just the circle and a couple of other parts and put things back together.
So far things seem to bee in pretty good shape. A little cleaning, lubrication & adjustment and I should have a good T3 out of 2. Now I need to do some research to see if it is possible to replate the glass circle, but that is for another day,
Hey, that is cool! Good on you. If I remember correctly, the Swiss had a process for engraving the glass, that nobody else had, and that is why they got the GOVT contracts to supply the US Army with their instruments.
I have a feeling that somebody, somewhere knows all about what you are doing, and can tell you more.
N
Jerry Penry & I wrote an article about this published in American Surveyor. You can check it out here:
http://www.amerisurv.com/PDF/TheAmericanSurveyor_PenryIngram-TheAmericanTheodolite_Vol10No9.pdf
once again, dave, cheers and congrats. i still don't have the heart to pop the cover on most instruments. i don't think i can make it down there with the t 3000 any time soon, will probably pack it up and ship it to you
Dave- A very interesting project! The pitting seen on the circles almost looks like an old fingerprint that damaged the silvering. Your article on the Gurley theodolite and the manufacture of the glass circles gave me some ideas. The original process of masking the circle, scratching the mask, etching the glass, and then filling the etched marks with Lead Sulfide gives us some hints on how to restore the circles.
I AM NOT AN EXPERT, but... I suspect the big problems that will become apparent when attempting to resilver the theodolite glass circles will be inadvertently removing the lead sulfide from the etched lines and numbers. My first thought was to just clean the circle with alcohol and distilled water and apply silver plating chemicals directly to the pits without actually stripping off all the old silvering. This might be "good enough" although I suspect that there would be some defects. To strip the entire circle, you might be able to strip the silver with the silver remover, gently rinse the circle without scrubbing out the lead sulfide in the grooves, apply polishing rouge slurry on a piece of plate glass and gently polish the surface of the circle by placing it face down on the plate glass and using a gentle circular motion with light pressure applied to the circle. This would possibly preserve the lead sulfide in the groves and numbers and allow the adjacent glass to be polished enough to resilver. If you had a dissecting microscope or a really good hand lens and light you could inspect your progress. If you had to "re ink" some of the marks and numbers, you might be able to refill the etched areas and then polish off the excess using the glass plate and rouge. I suspect that it would take something more substantial than a black sharpie to fill the etched areas. Lead sulfide was used as a pigment in the past, although I don't know the application process. If you were able to get the entire surface cleaned and polished, resilvering is probably the easy part! I read the instructions from the article linked below and the information seems pretty good- particularly on using the warmed distilled water. I don't see that there is the kind of backing paint on the circle that one sees on a mirror, although I could be incorrect. One might have to sacrifice a circle to develop the technique to strip off the original silver finish without removing the leas sulfide. Just thinking...
This is a link to a mirror silvering instruction guide that also has some interesting articles at the end on antique glass that you may find interesting.
http://www.resilvering.com/files/Download/DIGITAL%20MIRROR%20MANUAL%202013%20-%20TIM%20INMAN.pdf
Other links of interest from the mirror resurfacing home site:
www.historicinteriors.com/Resilvering.html
http://www.historicinteriors.com/Resilvering.html
http://www.historicinteriors.com/shop/viewitem.php?groupid=0&productid=10
http://www.historicinteriors.com/shop/viewitem.php?groupid=0&productid=0
TimH
