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c. 1865

SIGNED:J.Zentmayer, Maker, Philad' a  



Zentmayer Grand American Microscope Zentmayer Grand American Microscope

This page depicts images of my Zentamyer Grand American Microscope before and after a major restoration. Such an instrument in fine condition is both hard to find, and usually beyond the financial resources of many potential collectors. This was true for me, and so I bit the bullet, purchasing it at a relatively low price at an auction, but realizing it was in very poor condition. I hoped some day to restore it and that time finally came in the first few months of 2018. This page is about how it was restored. Unfortunately I did not have enough time to photograph the steps along the way, but will try my best to describe them.

Before starting a project like this one must have the proper tools and supplies. Ordinary screwdrivers for instance, will not work. You need them to have both the proper thickness and width of the blade. A reasonably satisfactory set is one used to dismantle old firearms and my set was made by Brownells of Montezuma Iowa. But this set is only good for relatively large screws. For miniature screws, of which there were many on the Grand American(GA), a large variety of high quality screwdrivers were needed. In some cases, I just had to buy extras and grind them to the proper size, both in thickness and width. Old brass screws are often tightly frozen in place and you need a bottle of Kroill and an eyedropper to apply it; you also need patience. The oil must be allowed to soak in, sometimes for days, and sometimes for several reapplications. If one tries to force these screws before they are loose enough, they can break and you are faced with removing the stub, redrilling the hole, retapping it, and finding or making replacement screws. Finding authentic replacements is almost impossible so you will need to make them or settle for something that is not authentic-looking. In fact, most of the screws on this microscope are non-standard threads. Although I have a threading attachement for my miniature Sherline lathe, such apparatus cannot easily apply a thread to a tiny screw, and dies for tiny screws are difficult to find in unusual sizes. Some simply do not exist. Fortunately I did not break any screws on this scope, and none were missing.

In addition to screwdrivers, sometimes other tools will be needed. Some of these I had to make. Some are easier to make than others. In some cases its simply a matter of figuring out how to use a tool without damaging the part. A piece of thick leather under the jaws of a plier for instance, may prevent damage. You will also need tarnish remover, and sandpapers with grits from 220 to 10,000 or more. You will need a good detergent like Dawn dishwashing detergent. You will need as pure and clean a methanol as you can find, and good quality shellac flakes. Acetone and Butyl rubber gloves (others will be dissolved by acetone), may help too. I soaked things in metal containers away from flames and sparks. Stiff plastic brushes of various sizes, cotton tipped swabs, rags, toothpicks, wire brushes, and artists paintbrushes are other neccesities. Other supplies are mentioned as we go further.

The first step in restoration is to completely disassemble the instrument. During this process you must be careful to make notes and you may need to take photographs before starting to remove parts and during the process. I did not need the photos, but highly recommend you take them unless you have a very good memory for such things. You need to do your work sitting down and comfortable. You need to be very careful to catch screws and other tiny parts so they do not get dropped into the carpet or lost in the garage-I have spent many hours sometimes trying to find a tiny screw I had dropped! Once you have removed the parts from some region of the microscope, you need to put them in little containers or zip-lock bags carefully labeled. With a microscope like this, there are too many screws to be able to keep straight which ones go where, I followed this practice of using labeled ziplock bags throughout. Remember that screws that look similar may not have the same exact threads or even the same diameter.

The rest of this article will describe the actual process of restoring this particular instrument, though much of what I will describe applies to any similar antique instrument.

There were two major issues with this microscope when I obtained it. Certain moving parts were not functional, and the finish had deteriorated beyond what could be accepted without completely refinishing. Both issues were approached simultaneously. The first step is disassembly and I removed every screw and every fitting possible, short of needing to desolder.

The main optical tube was racked up and off the limb as was the substage assembly. The eyepiece was removed and the objective unscrewed from the nosepiece. The gliding slide holder was likewise removed by loosening the ivory-tipped tension screw. The optical tube had its rack attached by screws and under the rack was another metal bar held in by additional screws; both were removed for proper cleaning. The rack should never be lacquered. The main optical tube had a nosepiece fine focus mechanism attached to it by very tiny screws. After proper treatment with Kroill, the fine focus assembly was removed. This left the main optical tube ready for refinishing later.

The fine focus assembly was completely frozen and unusable. Although removing the little support for the fine focus knob was simple, disassembly and loosening of the fine focus assembly itself was one of the hardest parts of this project. It took numerous treatments with Kroill and hand work to get the parts functional again. The part into which the objective fits was full of corrosion and verdigris and required alot of sanding with fine sand paper to reach a point where it would slide easily inside the fine focus assembly. This brings us to the use of sandpaper in restoration.

When one is faced with either a badly fitting part, or one which needs to be re-lacquered, one needs to remove the tarnish and also any major imperfections, which may require, in addition to tarnish removal, sanding. Following superficial tarnish removal with a suitable chemical, the next step for much of a project like this, is sanding. This is done by hand using progressively finer sandpaper. What most people would think of as 'fine' sandpaper (220 grit) is 'coarse' when it comes to doing this work. It was rare that I turned to anything coarser than 220 for any part of this project. In fact it was just the beginning. Sanding is done in the direction of the machining. Progressively finer papers are used until the desired look is apparent. For the main optical tube, this may be quite fine-5000 or even 10,000 grit to get the proper appearance. A buffing wheel is generally avoided, except perhaps for the main optical tube where it might be appropriate as a last step if done carefully. Electric sanding is generally avoided so as not to round over the sharp edges produced by machining. This is a lot of finger work, and can lead to stiff painful fingers, especially as one gets older. It has stopped many a restorer from continuing their work as they aged. Finger exercises may help these symptoms but not eliminate the problem completely. Sanding of this type is generally done with the paper (or cloth for finer grits) fixed to a flat sanding surface, again to avoid rounding over flat areas. But the sandpaper is used free hand on rounded surfaces, and there are exceptions to these rules as you will see.

Sanding curved surfaces again should follow machining marks if feasible. This is best done by figuring out a way to spin the part while sanding. This is what I did for the knobs, pillers, and the base plate the pillars fit into. Lacquering these parts is also done with them spinning. I used my Sherline lathe for this, but a variable speed drill could also be adapted for the purpose. It may be neccesary to produce specialized parts to hold the part being restored to avoid damaging its finish with the chuck of the lathe or drill. Very slow speeds are used when doing these operations. In restoring this microscope I did have to make some of these fixtures. While we are on the subject of the knobs, you will notice they are all knurled (referred to as milled in the older and British literature). These knurlings cannot simply be sanded-it would ruin their sharp machining marks. They are cleaned using a revolving brass wire brush on a Dremel tool or an electric drill, with the bristles traveling in the same direction as the cracks between each tooth of the knurl. Occasionally I have had to resort to steel brushes-but this is risky. This process may not get them back to a shiny finish but the crud is easily removed to the point they can be tastefully relacquered. The knobs are not rotating on a lathe during this operation, but rotated by hand as each area is sequentially cleaned.

As I started to disassemble the microscope, I found the pillar plate would not rotate smoothly on the tripod. This was because of thick frozen grease inside the fittings. This needed to be carefully scraped out, then loosened further with Kroill and finally wiped clean. It then required some sanding to remove the corrosion and verdigris so that the parts would revolve properly. Relubrication was then neccesary with a thin silicone grease or 3 in 1 oil, depending on the part. The inside of these fittings, like any other contact point of a moving part, should never be lacquered. As I mentioned earlier, after removing the pillars from it, the exposed surface of the revolving plate supporting the pillars was sanded on my lathe.

The pillars were next but before doing anything with them they needed to be removed from the limb and trunnion support. This was not as easy as simply removing the tension caps. The axle supporting the pillars is quite complex and was very tight. It was not the outside tension adjustment that you can easily see, but rather the assembly beneath those that was tough to loosen. Zentmayer went to incredible lengths to make that assembly sturdy and unlikely to loosen despite frequent adjustments of inclination. Unfortunately that left me with some parts extremely difficult to disassemble including some tiny screws which were inserted through the limb into the axle of this support. Again, the outer visible surfaces of the supporting parts on the axle were cleaned and relacquered on the lathe.

The stage was attached to the limb via a saddle with a large single screw passing through both the saddle and the limb from the side. Once this was removed, the entire stage assembly came off easily. The stage assembly however is quite complex. The saddle is attached to the outer support ring of the stage by four hefty screws. The slide glider was removed from the rest of the stage and required a lot of tiny screwdriver work to take apart. The support for the ivory-tipped tension screw was attached by two screws underneath the stage. The calibrated rotating stage was a bit of a challenge also. There were many concentric rings, each with 8 small screws to remove. Furthmore, I discovered the stage would not easily rotate even after thorough cleaning. On close inspection, the reason became clear; the edge of the silvered top stage plate had been damaged and burrs were digging in as it was turned. This required careful flattening of the bottom surface with sandpaper before it would rotate properly. With that flattening and proper lubrication, it rotates beautifully now.

The swinging tailpiece for the mirror rotates on a screw passing through a protruding part of the limb. The tailpiece is hollow and has a gimbaled mirror housing containing a concave mirror on one side and a flat mirror or the other. This mirror housing is the one piece that I could not disassemble no matter what tricks I tried. I would have had to ruin the part to open it and so I left well-enough alone, so the mirrors are unrestored.

Before lacquering, the parts that were originally silvered need to be recoated. The original method used arsenic-not a wise idea. Today I use a cheap low voltage silver plating kit. In order for this to work properly, the chemicals must be fresh-do not buy your kit months or a year in advance. If it does not work, call the company and demand a fresh container of plating solution. In addition,all traces of grease and tarnish remover must be carefully removed with detergent, solvent or preferably both, prior to plating, and you need to be careful to plate only the areas you want plated. Silver plating needs to coated with lacquer or it will tarnish rapidly. On this project the calibrations for the foot rotation, the goniometer stage, and the fine focus wheel have all been resilvered.

Once all the parts were disassembled, the tarnish removed, and parts sanded, it was time to relacquer. The first step in this process is the removal of tarnish and sanding as I have already described above. The next step is to remove any trace of grease, oil or dirt. This is not easy, and I have not found a perfect way to do it. I have tried several. One is to use boiling hot (not tap hot) soapy water with Dawn dishwashing detergent. Stiff plastic brushes are used to make sure the cleaning is thorough. After rinsing with plain water, the part is then cleaned with Acetone (wear special Butyl gloves), or pure methanol until the cloth or swab you are using no longer picks up any dirt. Finally the lacquer can be applied.

Modern 'lacquer' is NOT real lacquer and requires special (dangerous) chemicals to thin or clean. The original lacquer was made from pure methanol and shellac and that is what I used. You cannot just use any old methanol-it must be high quality and almost pure. Making the lacquer is not easy. Shellac comes in "flakes" which are more like rocks! It has to be pulverized and added to the methanol and the mixture agitated as the shellac gradually dissolves. It is generally let sit overnight and then it must be filtered through cheesecloth several times to obtain a product free of contaminants. It is then diluted 1:4 and 1:2. The more dilute solution is used as the first coat. In some cases the most dilute coat is all that can be used even when recoating. It is best applied in very dilute multiple coats rather than single heavy coats. You cannot use an ordinary brush-you need the highest quality artist's brushes. The proper technique is to use small amounts of lacquer on the brush and use consistent unidirectional continuous strokes on flat surfaces. As I mentioned above, the round parts are lacquered while spinning. Depending on the temperature of your lacquer and the part, you may need to cool the lacquer, heat the part before lacquering, or both. Once prepared, lacquer in the liquid solution is sensitive to air, light and heat. It will go bad in weeks to months if kept out of a refridgerator. I keep mine in a special little fridge just for that purpose. Most lacquers of the nineteenth and eighteenth centuries were not clear, so I use the darkest variety of shellac I can find. Always use dewaxed shellac on instruments. You can always apply museum wax over the lacquer. I got my shellac and some of my other supplies from

Reassembly should wait until at least 3 days after the last part was relacquered, longer if the humidity is high or the temperature low. I also learned the hard way that the order of assembly makes a difference. Putting the pillars on the inclination joint first, was much simpler than putting them on the bottom plate first which only led to problems.

When you are done, the instrument should not look like it was made yesterday. I even leave a few blemishes and do not refinish parts that are not seen without effort; in this case I did not refinish the saddle, for instance. It should also not look like the machined edges have been polished off leaving rounded edges that simply look artificial.