Wooden slide rules can be divided into two major categories: hardwood with celluloid coverings that have the scales engraved and ink-filled, or more simple pressed wood rules with painted faces. I'll cover the higher quality celluloid rules first, as these are more common among collectors and users. They are also the more valuable, and thus require special attention to insure they aren't damaged in the cleaning process (please see the standard warnings on my general Cleaning Slide Rules page). Incidentally, there is a common misconception that many celluloid-covered rules are made of Ivory. Celluloid was invented in the later half of the 19th century as a synthetic replacement for the expensive (and hard to work with) natural Ivory. Virtually all makers used it exclusively from that point on. Case in point: K&E was only in the slide rule business from the late 1880s onward, and thus used only celluloid. In any case, I don't recommend any sort of cleaning for a rule that is more than a hundred years old, so Ivory really isn't an issue for us.
The first thing to do is verify that your celluloid covered rule has ink-filled engraved scales. If it doesn't, and the scales seem to be simply painted or printed on the surface, then I'd suggest you skip all but the first few cleaning methods listed below. Also (and with one potential exception further down this page), I'd recommend you never use any sort of cleaning solvent directly on the surface of a rule, or you are likely to dissolve the ink. In case you do, Larry Stewart, resident slide rule collector and graphic artist, has developed a method for restoring pigment to abused engraved rules. Drop him a line if you want to know the details.
The image up above is an old K&E 4081-3 rule that showed up in the mail in pretty bad shape. I cannibalized the cursor assembly and case for a similar vintage model, and decided to use what was left as a "tester rule" for examining different cleaning strategies (sacrificing its life so that the more worthy might live ...). The relative merits of each of these procedures is listed below, beginning with the mildest. You can click on the various portions of the rule above for a blow-up (150 dpi) of the affected area. For more a more detailed comparison of all areas, see the bottom of this section.
Aging-induced yellowing of slide rules can significant detract from their visual appeal. Mineral oil can sometimes be used to lighten the colour of the celluloid, although I haven't had much success with this method. Apparently, this works better on Hemmi branded rules that K&E, or so I've been told. It seems to be safe for the ink, but playing with oil can get a bit messy as you have to clean everything thoroughly afterwards. Never use any other kind of solvent, as you are likely to remove the ink as well. Personally, I'd skip this step and proceed with the more effective methods listed below.
A little diluted Palmolive in warm water is my stock-in-trade for cleaning all-plastic rules, and it can be quite useful for cleaning surface dirt off celluloid and aluminum facings as well. In fact, for Hemmi branded rules (like the Hughes Owens ones here in Canada), this is actually the recommended option for regular maintenance. I usually just gently rub the surface of the rule with a paper towel or soft cloth dipped in diluted Palmolive (for about a minute in the example above), and then wipe with clean water. Avoid immersing a wooden rule in water. It is safe to gently wipe the wood with a wet cloth, as this can help clean off loose dirt and dust, but always make sure you immediately dry the wood. Sometimes a Q-tip can be quite handy for problem spots, or difficult to reach corners. This is probably the safest option available to you, but it tends have limited results on poorly maintained rules.
Thanks to Larry Stewart for this little tip, I've found that a standard artist plastic eraser (like the white Staedtler pencil eraser shown on the left) can be your best friend when it comes to cleaning celluloid facings. You can pick one of these up at any stationary/office supply store, and it works like a charm to clean up all sorts of abuse and neglect. It is fairly gentle and, if you are patient, quite effective. The only problems that I can see are that it is more time consuming than the standard Scotch Brite method presented below, and tends to generate more heat for that very reason (although this doesn't appear to be a problem). Also, it doesn't seem to clean quite as thoroughly as a Scotch pad, but it is a close second. One bonus I've noticed with this method is that it tends to leave the "glossy" varnished look of never (or rarely) cleaned rules intact, whereas other more abrasive methods can take that away. If you are at all worried about damaging the rule, or if it still has that new-looking shine and it is only slightly soiled, then this could well be the best option for you.
The de facto standard among collectors for cleaning celluloid facings. K&E in fact recommended steel wool for regular cleaning of its rules, but most people prefer something a bit gentler (not to mention easier on the hands). I've experimented with various brands, and actually prefer one of the no-name nylon pads available around here over the actual Scotch brand scouring pad. Some of the cheapo brands can be all but useless though, so you might want to stick with the brand name if in doubt. These can thoroughly, and safely, clean an engraved celluloid rule faster than anything else I've seen. Beware of overly rigourous scrubbing, or spot cleaning, as this has the potential of disfiguring your rule. Once you start, you are best to continue over the whole surface of the rule to keep everything nice and even. With a little practice and care, this is probably the most effective method for regular maintenance of a slide rule. If they only knew what use their pads were being put to ... if they did, I'm sure they would add it to the packaging label, as I have on the image to the right.
It is amazing just how well this works to restore that ivory-white complexion to yellowed celluloid! Most people assume that sunlight has caused the yellowing of old celluloid rules, but this doesn't seem to be the case at all. This is especially an issue for K&E rules, which seem to suffer from aging-induced yellowing faster than other manufacturers. I suspect oxidation and pollutants such as cigarette smoke are more likely culprits, although tannins leaching out of the leather cases are another possible suspect.
Surprisingly, limited direct sunlight exposure also seems quite safe for the ink, although you shouldn't push the exposure time too much. I think the detailed comparison image of the different cleaning methods shown below illustrates the effectiveness of this method. That was after a 3 hour exposure to the early spring afternoon sun. Make sure you've cleaned your slide rule thoroughly before you attempt this procedure, or you are likely to simply increase the contrast between clean and dirty areas. As always, I would definitely have to say that less is more in this case - just like for your own skin, don't overdo the exposure. I have since attempted 4 hour exposure in the mid-day summer sun, with definite colour loss in the painted scales. Since the effect is additive, I would start with a short (i.e. one hour) exposure, and then expand if necessary.
Something new I've come across, thanks to a suggestion by Mike Markowski at the Yahoo slide rule discussion group. Designed to be an all-purpose metal polisher (or, more precisely, a metal tarnish remover), this material can have a dramatic effect on all parts of a slide rule. It 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 or in the garage with it!). While it polishes metal quite nicely, it also (surprisingly) seems fairly safe and effective at removing surface grime and yellowing from celluloid rules. In North America, you can usually pick it up at any major hardware supply store (I went to Canadian Tire for my tin).
On the right, I'm showing a "before and after" scan of the same part of the slide of my tester K&E rule after about a minute of gently rubbing with a piece of the Nevr Dull cotton. It was definitely more effective than either the white eraser or Scotch Brite pad on their own, given a roughly equivalent amount of rubbing. Remarkably, the surface of the rule seems even less scratched than either the eraser or the Scotch Brite treated portions. And even more amazing, it doesn't seem to have had any effect on the ink in the scales!
Given the unpredictability of the interaction of harsh solvents and slide rule ink, I'm generally not inclined to recommend this kind of approach. I certainly don't think it's a good idea for regular maintenance, despite the apparent safety on this round of testing. But if you've got a rule that seems beyond all hope, it's definitely worth a try. I'd suggest trying to give an even cleaning over the surface of the rule, and wiping off any excess residue that is left behind (a little warm water and lots of paper towel would be advised). And, of course, I'd strongly recommend you wear rubber gloves and work in a well ventilated area when using this stuff. Safety first!
Definitely the low end of slide rules, these were typically made as beginner rules for younger kids. Examples would be the Lawrence rules on my Miscellaneous Slide Rule pages or the K&E 4058W. Some of these rules have scales painted or printed directly on the wood (which generally results in poor contrast), while most typically have white painted faces with the scales printed on them. I doubt many collectors would be interested in using or restoring these rules (they typically only cost pennies when they were new), but I've found a white eraser works fairly well on the painted face variety, and can help restore a little contrast to a dingy surface.
A common problem with wooden rule is adjusting the tension so that the slide consistently moves freely (but not loosely) through the body or stator pieces. This is especially problematic since most types of wood expand and contract depending on temperature and humidity (try using a wooden rule on a hot, humid day and you'll see what I mean). Bamboo rules (such as those made by Hemmi or Ricoh/Relay) are typically superior in this regard, and also have the additional advantage of rarely needing lubrication as they are somewhat self-oiling.
Probably the best thing you can do for a wooden rule is simply keep it clean and well-adjusted for tension. I generally use a little diluted Palmolive to clean to the wooden grooves, and then wipe clean with water and paper towel or a soft rag (Q-tips can also come in handy). A small amount of a decent wood oil can then be used to help smooth everything down and keep it sliding well. Body/stator tension can be adjusted by loosening the tension screws, re-aligning everything, and then re-tightening with the appropriate gap. These screws are present on most American-made wooden rules (both simplex and duplex), but are less common on European or Japanese simplex rules. In these cases, you usually need to slightly "bend" the body of the rule length-wise to insure a snug fit for the slide between the stator pieces. I know it sounds horrific, but this is the recommended approach (there's actually a metal plate inside the body that is responsible for maintaining tension).
As for lubrication, I personally don't bother with it much on wooden rules. I certainly don't recommend waxing a wooden slide rule. This may look pretty, but the buildup of wax will impede the normal sliding action of the wood. It's possible a Teflon-based lubricant (which I use on my Metal Slide Rules) might help here, but I haven't tried that yet. A little wood oil is probably your safest best for cleaning the the sliding pieces. If you like, you can rub a little Pledge or wood oil on the back of desktop models (like my 4053 series) to help bring out the wood, but be selective where you apply it. My advice: just keep it clean and well adjusted for tension and you shouldn't have any major problems.
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