Virtually all slide rules from late 19th century onward contained movable cursors, or runners, with hairlines to facilitate calculations with the rule. On most desktop model rules (like my K&E 4053 series), the cursor assembly can be slid right off and easily cleaned separately. Once again, diluted Palmolive works well here, but I generally prefer diluted Windex to prevent streaking and to leave a nice shine. Be careful of using Windex directly on the paint-filled cursor lines on the inside face of the glass - you might actually dissolve and wipe off the ink! If you do, not to worry, these can be restored, as I'll get to in a moment.
Duplex rules (like the K&E 4081 series for comparison) can be easily cleaned without disassembly by sliding a piece of paper underneath the glass. With a little practice, I think you'll find that this picks up most of the dirt stuck there (you can also moistened the paper to pick up more, if need be). Diluted Windex also works well, but can easily dissolve the cursor hairline, so be careful. For more thorough jobs, I usually disassemble one side of the assembly and thoroughly clean both sides of each cursor glass. I find I often need to do this anyway to adjust misaligned cursor hairlines on duplex rules, but re-alignment can be tricky if the cursor end braces are not in very good shape. Be very careful about disassembling duplex cursors if you have any doubts as to the stability of the end pieces (see below)!
This condition is evident on many K&E rules made prior to 1950. Apparently, they used some form of plastic for the cursor assembly end pieces that easily disintegrates (known as the K&E Rotting Cursor Syndrome, or KERCS). An example of a KERCS-afflicted cursor from a 4053 is shown on the right. The bottom piece has already broken apart, and the top piece is full of stress fractures and wouldn't survive much handling (you can't really tell on the scan, though).
K&E realized this during production, and switched in the late 1940s to a far more durable plastic (which, interestingly enough, doesn't show the considerable "yellowing" of normal celluloid with age). If you have any doubts as to the stability of the end pieces, my advice would be to leave well enough alone and simply clean underneath with moistened paper - it works. If you are already in possession of a KERCS-afflicted rule, the odds of your finding official replacement parts at this point in time are slim. However, you can always cannibalize the cursor assembly from a later-generation K&E rule. For that matter, most Hemmi and Dietzgen duplex cursor assemblies will also fit, if you don't mind mixing manufacturers. Finally, David Crate has also developed prosthetic replacement parts for this problem ... he's even gone so far as to imprint them with K&E's name and patent numbers! Check with him for pricing and availability.
Cursor hairlines can also be restored thanks to an ingenious tip from Walter Shawlee. If your cursor is made of glass (I wouldn't dare try this on plastic), you can use a permanent marker, like a Sanford Sharpie or Staedtler Lumocolor, to re-fill the line. To do this, thoroughly clean the inside face that has the actual hairline engraving with diluted Windex, and let it dry. Vigourously scribble with the marker all along the area where the hairline etching is located (special bonus: you can use whatever colour you like). Don't worry about making a mess - you will clean that up later, but it is important to thoroughly fill the engraving. Give it a few moments to dry, and then begin gently wiping away the maker with diluted Windex and a paper towel. If you are careful, I think you'll find that the ink on the smooth surface of the glass comes off fairly quickly while the ink in the groove remains longer. Once you're satisfied with the results, let it dry a little longer and you are ready place the glass back in the frame and reassemble everything. Just make sure you are dealing with glass cursors before you start - no guarantees on more porous materials.
One potential problem with this method is that permanent markers are generally somewhat translucent, compared to the original opaque pigment. If this is a concern for you, Larry Stewart recommends an alternate method whereby you rub artist's full strength acrylic paint into the groove using a tissue, and then wipe off the excess once dry with a moistened Kleenex. I haven't tried this approach yet, and I suspect it might require little more finesse than the lumocolor method.
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