Before and After

This evening I sat down to start restoring a fine old Dring & Fage slide rule, and I finally remembered to do something I've been meaning to do for a long time: show a “before” and “after” photo.

This particular slide rule dates back to the mid-1800s. It's an unusual style, with two slides on each side of the body. The condition of this example is superb, with almost no damage or even signs of wear. The only notable problems are (1) it is filthy, as you might expect from something 150 years old, and (2) the parts fit too tightly to move smoothly.

In the photo above, the top piece is a segment of an uncleaned slide and the bottom piece is a segment of a cleaned slide. This rule has four slides plus a rather large body – it took me 45 minutes to clean the first slide, so the whole rule will probably take about 5 hours to clean.

The cleaning process is simple, but tedious: I soak an old washrag, fold it into quarters on my worktable, sprinkle some Bon Ami onto a corner of the washrag, and then carefully scrub one little piece of my work at a time. The trick is to scrub hard enough to get off all the soiling, but not hard enough to do any damage to the finish – the only way I know how to do this is trial-and-error, starting with very little pressure and working my way up.

Once I've thoroughly scrubbed the entire workpiece, I inspect it carefully for any firmly adhered stuff. These chunks I carefully work off with dental picks. This particular piece had a few specks of something that looked like wax, plus a few paint droplets. Most of the pieces this old that I restore have far more adhered junk.

The last cleaning step is to rinse the workpiece, which is more of a challenge than it sounds. Slide rules like this have very finely engraved lines on them, and if they're made from certain woods (such as mahogany) open pores. These engravings and pores accumulate soap, feldspar powder from the Bon Ami, and fine particles of whatever the slide rule was exposed to in its life. All of this stuff must be rinsed out, and usually the force of the tap water is not enough – I also have to scrub out all the pores with a very fine, stiff-bristled brush. That last bit can be quite tedious!

Once I've cleaned the entire slide rule, and all the parts are thoroughly dried, I'll apply several coats of Johnson's Paste Wax. That will protect any bare wood that's been exposed, and it will fill in small pores and scratches. I let the wax dry for a few hours and then polish it up with a shoe-polishing rag. In the photo above, the bottom slide has been cleaned but not waxed; it will be a little darker and much shinier once it's been waxed.

One interesting thing I've learned about these old wooden slide rules is that they are impervious to water. When I first started restoring these instruments, I was afraid to get them even slightly wet – I figured that would raise the grain and ruin the finish. After a few accidental soakings caused no harm, I started getting bolder about the use of water – and now I don't hesitate to put a 200 year old instrument directly in the tap water. The only slide rule I've ever damaged with water was an unfinished wooden rule made by Lawrence (an American manufacturer of crude, low-end slide rules), and even that one was easily fixed with a little fine sandpaper. The combination of water and Bon Ami cleans the old wooden instruments better than anything else I've ever found…