Most all-metal slide rules were made of aluminum. There are a few exceptions to this, but other materials tended not to work very well. Case in point: early Pickett magnesium rules tended to quickly break down and take the painted scale faces with them. I certainly don't recommend any sort of cleaning on a magnesium-based rule, since you are likely to only exacerbate any problems that may exist. Other types of metal (or various alloys) may also have been used, but most metal slides rule you'll come across will tend to be made of aluminum.
And that's not such a bad thing! Aluminum rules can be of very high quality, and I've found that they often resist changes in temperature and humidity better than wooden models. They also seem to age better than other materials, and seldom warp (especially a problem with wooden rules that weren't properly stored). The main age and storage related problem with metal rules is built-up corrosion along the sliding surfaces that can make it difficult to get the slide moving again. If you are having this difficulty, try reducing the pressure on the end braces by loosening the screws (if present, which they generally are). Also, never hold the rule in the middle of the body, or your own hand pressure may be inhibiting movement. Once you get everything freely moving, it's time to take a look at cleaning and lubrication options.
A last point to keep in mind is that the scale markings tend to be directly printed on aluminum rules (using a photolithographic technique in Pickett's case, for example). So be careful using anything even the least bit abrasive on the surface of the rule. You are likely to severely scratch, or worse completely remove, the scales this way (see further down this page for an example of what I mean). Incidentally, another common problem on badly stored rules is the "chipping" away of the painted faces due to corrosion (especially along the edges). There is nothing that I know of to reverse this problem, and any cleaning is only like to make matters worse. Proceed with extreme caution if this is the case with your rule!
Once you get everything sliding again, my stock in trade for cleaning aluminum rules is diluted Palmolive. I typically like to completely disassemble the slide rule (if possible), but this is not a necessary requirement. Some cursor assemblies (like later model Picketts) can be a bit fussy to put back together again accurately, so keep that in mind before you take anything apart. Unlike wooden rules, feel free to completely immerse the rule in a tub large enough to hold it. Depending on how badly stained the rule is, anywhere from 20 minutes to an hour should be more than sufficient for helping to lift off any stains and grime. When it comes out, simply wipe with paper towel, rinse, and wipe dry. Again, a Q-tip can be an indispensable tool here for hard to reach places (like inside the end braces). This usually works quite well, but as in the case of celluloid rules, don't expect miracles here.
Be wary of using solvents (like Windex, for example) directly on the face of the rule, as these may dissolve the markings. I've tested Windex on a small corner of a damaged Eco Bra rule, and it quickly started to dissolve the painted surface when rubbed with a paper towel. Personally, I'd never want to go any stronger than diluted dishwashing soap. Even cursor assemblies need to be handled carefully, especially since most are made of plastic on aluminum rules.
So, what do you do if the rule is so badly stained that diluted Palmolive alone isn't good enough? In extreme situations, you could try to gently scrub the rule as it comes out of the dishwashing soap with a wet Scotch pad (typically used on celluloid covered rules). Let me re-emphasize that I do NOT recommend this procedure, except in emergencies. A good example of which was a badly beat-up, old-style Pickett model 3 rule that I got "free" with another slide rule purchase. Figuring I had nothing left to loose, I gave this method a try, with the results depicted below.
At first glance, this seems to have been very effective. What you are looking at are before and after shots of the rule done at the same scanner settings for direct comparison. In fact, the original rule looked even worse than the scanner image (the back of the rule was in even worse shape, with definite signs of some sort of unidentifiable, but unmistakable, mold). Simply put: you felt like wearing gloves before touching it, and then immediately washing you hands afterwards. I think you can see from the first set of close-ups below that many areas, including those with localized heavy stains, cleaned up quite nicely.
Even the cursor came out quite well, with relatively little scratching evident. The metal end braces and cursor assembly also cleaned up well, although that doesn't come across well on the scanner (shiny things never do). So what's the problem, you might ask? Well, check out the image below to see what happened to the first piece of the slide rule I started cleaning with the scotch pad.
I take it my point is made? The rest of the rule came out quite well, once I had learned to curb by enthusiasm, but alas the rule is ruined none the less. Unlike celluloid, scotch pads require a VERY gentle hand on painted surfaces (if you even dare try them, of course). Even carefully cleaning off the grooves of the slide resulted in some loss of the scale markings at the edge, so be extraordinary careful here. 'Nuff said?
Probably the most common problem with old aluminum rules is getting the slide moving again. Also, unlike wooden or plastic slide rules, aluminum rules require periodic lubrication to function smoothly. Most manufacturers recommended a very low-tech solution to this problem - petroleum jelly (i.e. Vaseline). Basically, all you have to do is add a little bit of Vaseline to the slide edge and then work the slider back and forth to lubricate the rule. The same goes for the top stator groove that the cursor spring slides back and forth along. This approach actually works quite well, but go easy on it. I once had to clean off what seemed like half a ton of some mucked-up lubricant from an Eco Bra rule - yuck! Lubrication is no substitute for proper cleaning.
An alternative modern solution that I've found to be quite effective is the newer "dry" Teflon-based lubricants. The term dry is a bit misleading, since these are actually in liquid form when you apply them (they subsequently dry and leave a Teflon film behind). There are numerous brands available, and one of the most common places to find them are in the bicycle section of your local general goods store. Much better than oil or grease for lubricating bike chains, brakes, etc. Shown on the right is one brand I picked up for my bike, which I tried on an old beat-up Pickett to see what happens. It left the rule with a really silky smooth sliding action, better than anything I've seen with Vaseline (bonus: no mess once it dries!). I generally apply a reasonable amount to the slide grooves on a clean rule, and then working back and forth within the stators to fully coat the sliding surfaces. Immediately wipe away any excess on the face of the rule, and then remove the slide and let everything dry. I think you'll find this a far more efficient lubricant for aluminum based rules.
Update: I've recently been contacted by someone who tried using a Teflon lubricant on a Pickett rule, and it damaged his cursor assembly. Apparently, some of the wet lube spilled onto the plastic cursor, and it caused the plastic to blister! Although I haven't had any problems yet (knock on wood), I'd recommend you keep this in mind when experimenting with this sort of lubricant (you may want to remove the cursor assembly first, for example). Caveat emptor!
Incidentally, if you are worried about your Pickett slides "catching" as they move through the end braces - don't be. Pickett deliberately put little tension springs (basically small, flat pieces of metal) over the stator grooves under the braces in many of their rules. This prevented the slide from accidentally falling out of the rule, but it does make an annoying "click" every time it passes through. These can be easily removed to turn your slide rule into a "speed rule", if you so wish (in fact, it often happens accidentally when you go to disassemble a rule for cleaning). However, some collectors even like to put makeshift versions back into rules that are missing them! Live and let live, I guess.
Although hardly a common slide rule material, you'll still find the occasional novelty slide rule tie-clip that is made of silver (sterling, in the case of the rule on right). In cases like this, any standard silver polisher should do. I've recently adopted Nevr-Dull, which appears to be some sort of cotton wadding impregnated with an organic solvent (smells something like a cross between turpentine and acetone - I suggest you work outdoors with it!). Oddly enough, it seems to be quite safe for engraved celluloid rules, but I wouldn't want to push that too far. In any case, it cleaned up my tarnished slide rule clip quite nicely. The rule is actually quite shiny as a result of this treatment, but I captured the image in low light levels to reveal the detail without the glare. A bit time consuming to clean thoroughly (given the small scale you are working with, literally as well as figuratively), but worth the patience. In North America, you can usually pick this up at any major hardware supply store (the Nevr Dull that is, not the tie clip).
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