Yesterday I received a package with two slide rules in it. They were from an auction I’d won on eBay. One of the slide rules was a Keuffel & Esser model that I wanted to add to my collection; the other was one that the seller could not identify (not even the maker), and from the lousy photo in the listing didn’t look very interesting.
So I was very surprised when I opened the package to find that the unidentified rule was actually the more interesting of the pair. There was nothing whatsoever wrong with the Keuffel & Esser, mind you — it’s just that the other one was a real prize. It was extremely grubby, and I wish now that I had taken a “before” picture so I could show you what a good cleaning can do. But even when it was dirty I could see that it was something special: a very old Dietzgen, almost certainly from the first decade of the 20th century.
The first thing I did was to search through all my reference books, and on the web, to try to positively identify the model. I failed; the closest thing was a Dietzgen slide rule scan I found at Mike Konshak’s wonderful online slide rule museum. Mike has this identified through the “3746A” that is stamped into the bottom of the rule. If that identification is correct, then the rule I have is most likely a “259A", as my rule has that number stamped in the same place as the rule Mike shows with the “3746A”. However, I’m suspicious that those stamps are actually serial numbers, and not model numbers — as I could find no references anywhere to a model 259A (or 3746A, for that matter).
So, I’m left wondering just what I’ve got. And I’d appreciate any help, or even wild speculations, that anyone might have (please use the comments here). At right are thumbnails of 300 dpi scans of the slide rule (after cleaning!); click on any of them to view them (warning: they’re big!). Worth noting is that the wood this Dietzgen is made from isn’t the usual mahogany; it is distinctly lighter in color, a sort of honey colored wood. I have a couple of other slide rules made from pearwood, and this looks exactly like them. The scan on Mike’s site looks like mahogany — but looking at my scans, the color didn’t come across very well, and perhaps it didn’t on Mike’s scan, either.
Boring notes on cleaning:
Most of the cleaning I did in a very low-tech fashion — I used very a dilute dishsoap solution, lint-free Kimwipes, and lots of elbow grease. A century’s worth of crud (whose components I really don’t want to know) had accumulated in every nook and cranny on the rule, even in the engravings. The slide’s motion was very difficult before cleaning, and I was worried it was in need of sanding; afterwards it was smooth as silk. Crud, and lots of it, was the only culprit.
I used a soft-bristled toothbrush to clean the engravings — tedious, but the result was crisp, black markings instead of the slightly vaguely defined, grayish markings it started out with. The biggest crud-removal challenge turned out to be the cursor — the inside of the slides had a millimeter or so built up. I soaked it for an hour in the soap solution, and then the toothbrush did the trick.
I always worry that the water in the soap solution will soak into these old wooden rules. My standard procedure with them is to use a very sparing amount of the soap solution, and to wipe it off immediately. This time, however, the crud was so thick — and the wood was clearly not being affected — that I felt safe using much more soapy solution (though still drying if off very quickly). There were no ill effects at all.
After this cleaning, there were still some yellow spots on the face of the rule. Rubbing my finger over them very lightly, I felt (or imagined that I felt) a slight bump, as though there were some substance on the surface. These spots were quite unsightly, and detracted a lot from the appearance of this otherwise very fine old slide rule. I decided to try abrasion, and I chose the finest grit abrasive I have: some 12,000 grit Micro-Mesh. I used some of the soap solution for lubricant, and my finger instead of the foam pad they give you (as I wanted to be able to apply more pressure). In about 10 seconds, those yellow spots were history — and better yet, there was no telltale texture difference on the spots I cleaned. Hooray for Micro-Mesh!
