Naturally enough, most manufacturers recommended that rules be stored in their cases when not in use. However, I know a lot of collectors who prefer to keep the rules stored separately in sealed plastic bags in acid free boxes, as they worry about the acids in the cases slowly aging the surface of their rules. In any case, most collectors agree that a rule is not worth much without its corresponding case, so you'll probably want to keep the case in good condition as well. Often cases are found in pretty poor shape ... which is of course the point, as they were always meant to take a beating in order to protect the rule.
I personally like to display the rules in their cases, and actually like the particularly beat-up ones as these seem to tell quite a story. In most cases, I would simply recommend keeping them clean and not trying to hide their age ... they can be beautiful collectibles in their own right. But as many of these cases were made of high quality leather, it seems a shame to let them languish in poor condition. Although there is a lot of variation in cases styles and materials, some simple rules for dealing with cases can help bring back neglected specimens.
Shown above is a old beat-up leather case for a K&E 4081-3, circa 1940, that I decided to turn into a "tester" experiment. Clearly, the case was somewhat neglected over the years, and has aged to a dark brown finish not uncommon on cases from this era. What I've done is treated the lower portion of the case with a three-pronged leather cleaning strategy, with fairly good results. I'm planning on restoring the rest of the case now, and then matching it to a suitable rule from the time period. The first step is to stop by any shoe repair or leather works store and pick up a couple of items, as itemized below.
The first thing you'll need is a good quality "saddle soap" for cleaning leather products. Often found in an oversized tin similar to shoe polish (see step 3), this material has a waxy feel that lathers up slightly when wet. I like to use a moistened sponge and apply a generous amount to the case with vigourous rubbing. Very quickly, I think you'll see lots of surface dirt and grime lifting off (you may need to rinse the sponge out a few times). This is undoubtedly the most important step in restoring the colour and condition of the original leather.
Thanks to a tip from Larry Stewart, I've also begun re-conditioning the leather after cleaning. Although Larry uses a specific paste recommended by antiquarian book binders, I've found a similar product available from the same manufacturer as the saddle soap above. Just ask the folks at the leather goods store for what they carry in stock for conditioning and moisturizing leather. Typically a white, milky, suspended solution, apply it fairly liberally across the leather and work it in a little with a soft cloth or paper towel. Let the stuff dry, and then buff with a soft towel. If it gets absorbed very quickly (i.e. if the case was very brittle and dry to start with), you'll probably need a second application to really saturate the leather. Larry warns against using too much, however, as this overly softens the leather (indeed, this stuff is often recommended for leather baseball gloves). In the case of the specimen above, it needed all the work it could get, so I was pretty generous (you can still see the severe cracks in the leather that you really can't do much about now). I recommend that you leave a slide rule inside to case while cleaning, so as to help prevent collapsing of the sides in case you over soften the leather.
The last step, which is optional, is rubbing a little neutral shoe polish into the case (let it dry from the previous treatments overnight first). Just follow the manufacturers instructions here, to whatever your personal tastes are (I typically don't like them too shiny). Alternatively, you can try rubbing a little Pledge into the leather with a soft cloth, but I don't recommend this as it could lead to a waxy build up and be a bit too slippery to handle. And besides, if you've gone to all the trouble to prepare the case so far, while skimp out on the last step?
Many synthetic cases come with leather closing flaps, so the strategy presented above still applies to those areas. As for the rest of the case, I usually just use a moistened cloth to wipe off surface dirt and grime. In extreme cases, a little diluted Palmolive might also help. Once dry, you can try rubbing a little Pledge into the material, as this helps to bring out a bit of a shine (just go easy on it, or the case is likely to slip out of your hands!). If anyone has any better ideas for dealing with these kinds of cases, please drop me a line.
Another problem that arises fairly often with old cases, especially those store separate from their rules for any length of time, is the collapsing of the case sides making it difficult to get the rule in or out. This is especially a problem on the form-fitting synthetic K&E and Hughes Owens cases. To make matters worse, these cases can become quite brittle with age, leading to irreparable damage if you try to straighten them out too forcefully. Walter Shawlee has developed an ingenious method for restoring these cases using a little steam from a tea kettle to soften the material so that you can force it back into proper shape. Check out his Slide Rule Universe site for more details. My only comment here is to make sure you don't hold the case under the steam too long or too directly, or you might burn the leather flap a bit (not to mention your hand!).
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