Repairing Slide Rules

Should You Repair?

We are all familiar with the scene from the Antiques Road Show where the appraiser told the owner that his colonial-era table was worth $25,000, but if he had not refinished it, it would have been worth a quarter of a million dollars.  What will repairing your old slide rule do to its value?

Repairing a broken slide rule will increase its value.  Refinishing it will not.  By “repairing” I mean removing graffiti and stains, replacing broken parts with authentic OEM parts, re-coloring hairlines or surface engraving to their original color, replacing lost or damaged screws, gluing down loose celluloid surfacing, re-sewing loose stitching on cases and other repairs that return the rule to its original condition.

Replacing a broken cursor with a cursor from a different maker creates a "Frankenrule."  A Frankenrule is more useful than a rule with a broken cursor but it is less valuable.

Here are some other "repairs" which adversely affect the value of a slide rule:
o   Re-coloring the hairline with the wrong color.
o   Re-sewing a case with a sewing machine.
o   Re-dying the case.

Cleaning the Slide Rule

An initial wipe-down with water, Windex, or very dilute dish detergent is good.  It might be acceptable to soak an all-plastic rule (I have never tried) but I avoid anything more than the most superficial surface dampness or wood or bamboo rules.  I keep a small spray bottle of Windex with my repair tools and spray directly onto the rule and wipe with plain Kleenex (not tissues containing lotion).

For serious cleaning, you must remove the cursor.  Then clean the body metal parts and the back before cleaning the body of the rule.

Metal parts: The metal end brackets on duplex rules can be polished with any non-abrasive metal polish.  Metal polishes will leave dirty stains on the celluloid body.   Those stains can be removed later but they can be prevented in the first place by covering the celluloid next to the brackets with masking tape.

Closed-body rules: If a paper or plastic table is glued to the back of the rule, leave it alone.  If the back table has come unglued from a metal plate, scrub the glue off the plate and polish it with steel wool.  Re-glue the table with Instant Krazy Glue.  (Don’t glue the table if it wasn’t originally glued--your goal is to restore the rule to its original condition.)

General yellowing:  I have tried “Cascade Plastic Booster” (designed to remove yellow and orange stains from plastic ware), “Mr. Clean Magic Erasers” (designed to remove crayon and ballpoint stains from painted surfaces), and perhaps a dozen other products.  None of them works as well as simply leaving the slide rule out of its case for a couple of months.  I have also tried Oxyclean spot remover.  It removes slightly more of the yellowing than any of the other products but it also removes red scale markings more effectively than it removes yellowing.  I don't use it anymore.

It is my suspicion that leather cases are the principal cause of yellowing.  New rules that have been stored in leather cases for years acquire some yellowing even if they have been sealed in factory plastic bags within the leather case.    If you are in a hurry, you can try bleaching a yellowed rule in sunlight.  I have exposed test rules to bright sunlight for up to a week; the yellowing bleached out quickly but so did a little of the red lettering.  My sunlight tests were on high quality Hemmi rules; I would fear total loss of red from cheaper rules.

Ink stains, embedded dirt:  Engraved slide rules from quality manufacturers can take a surprising amount of abrasion.  I get excellent results with 0000 (finest grade) steel wool.  Walter Shawlee swears by ScotchBrite pads but they have not worked for me.  Abrasion works only with engraved rules—generally that means celluloid on wood or bamboo.  Metal and all-plastic rules are printed (sometimes photographically) and abrasion will take the markings off as quickly as it removes the stains.  Some high-quality plastic rules are engraved but I have not tried these techniques on them.

Clark McCoy, in the International Slide Rule Group, says he has had good results with 320 grit sandpaper, stroking the sandpaper back and forth in the long direction of the rule.  320 grit is critical; coarser grit leaves scratches and finer grit polishes without removing stains.  This is very aggressive cleaning but it makes a lot of sense; the final step in the original manufacture of engraved slide rules is to sand the surface to remove ink spills and other defects.

Abrasive erasers such as "Pink Pearl" are excellent for preliminary treatment of pencil marks before a general cleaning with steel wool or ScotchBrite.

For non-abrasive removal of ink stains on celluloid surfaces Daniel Fournier recommends 99% rubbing alcohol followed by Novus plastic polish.

"Nevr-Dull:" Nevr-Dull is a metal polish.  It comes as small can of treated cotton wadding which you tear off and rub on you slide rule.  It takes a lot of elbow grease but is quite effective at removing stains and yellowing.  It leaves a shiny patina like a rule that has been handled a lot.  I prefer it to steel wool on old rules that have acquired a patina.  I have not tried Nevr-Dull on anything except engraved celluloid rules but I suspect it might be good on plastic and metal rules.  NevrDull was recommended by someone from the International Slide Rule Group; I regret I don’t remember who.

Ron Spolar recommends cleaning with "Scratch Out" on a paper towel stretched over your finger.  Scratch Out is a clear coat scratch remover for automobiles.  He says it cleans well and even removes yellowing.  I haven't tried it yet.

Wax:  I do not recommend waxing slide rules.  If, after all my cleaning, I find the surface sheen of the rule inconsistent—shiny in some places, flat in others--I just live with it.  I used to eliminate the flat spots by waxing the whole rule with good old Johnson's Paste Wax.  That produced a very attractive uniform sheen but ten or twenty years later the wax turned brown and made the rules look dirty.  I have not tried any of the special museum conservation waxes.  Ron Spolar recommends "Renaissance Wax," available from museum and conservation supply houses.  He says two or three coats will last forever and not show fingerprints.

Cursor Repairs

Back in the days when I used a slide rule every day I frequently removed dirt from under the glass windows by inserting a slip of paper between the cursor window and the body of the rule. Pressing the window against the paper while moving it back and forth would transfer the dirt to the paper.  That still works, of course, but nowadays I rarely have a need for a cursory cursor window cleaning.  For a thorough cleaning I disassemble the cursor.

Disassembling the cursor:  Unless you like crawling around on the floor looking for tiny screws, put a towel down or your work surface.  Use the largest “jeweler’s screwdriver” you can find.  Every hardware store sells them.

Cleaning:  Clean the glass with Windex and the plastic parts with 0000 steel wool—or whatever works for you.  A “detail brush” (big toothbrush with brass bristles) or steel wool works on the screws.  Thread the screws back into the plastic parts if you have trouble holding them while cleaning their heads.

Springs are usually "blued" steel.  Bluing is a thin surface treatment which cannot survive much abrasion.   A drop of thin ("3 in 1", etc.) oil on a Q-Tip is the appropriate cleaning tool.  Remove built-up "crud" with as wood or plastic scraper.  Wipe the spring dry before re-assembling.

Hairlines on glass windows: The simplest and surest repair you can make to a slide rule is to re-color a hairline that has disappeared.  Note the color of the original line; you will want to replace it with the same color.  Remove the glass and clean it thoroughly with Windex.  Scrape out the hairline.  (The reason the original color fell out is likely that there was dirt in the groove to begin with.)  Rub it with some colored substance.  I use a china marking pencil but crayons, felt-tipped pens (W. Shawlee recommends Staedtler-Mars Lumocolor; Daniel Fournier says that Sharpie Ultra Fine Berry is best for Hemmi cursors since the original color was cerise, not red.), shoe polish, and artist’s colors all seem to work well. Don’t worry about spillover, just make sure you get lots of color in the etched line.  Scrape off the excess color with a plastic scraper (credit card) or pasteboard.  Wipe the glass surface with a Kleenex moistened with solvent until the result is neat.  Windex is an appropriate solvent for most substances but use paint thinner if your colorant is oil paint.  You can begin wiping immediately in most cases but wait a few minutes for paint to set up before beginning to wipe.  If your first try is less than perfect you can fill in skips and wipe again.  If your first try is a complete failure you can scrape the color out of the hairline with a pin and start all over.

When replacing the window in the cursor, make sure the side with the hairline is next to the surface of the rule.  If, after you have reassembled the cursor, you can feel the hairline on the surface of the window when you scrape a fingernail across it, you have the window in backwards.

Hairlines on plastic windows:  Pretty much all of the instructions for glass apply to plastic as well but felt-tip ink and oil paint tend to penetrate into plastic.  If you use one of them the final solvent wipe will have to be a scrub with an abrasive plastic polish such as Novus #2 Scratch Remover.

Scratches:  Scratches can easily be polished out of plastic windows.   Plastic suppliers sell kits for polishing plastic.  (I use Novus Polishes.  They come is a set of three progressive grits.)  Polishing plastic by hand is practical but a cotton wheel mounted on a grinder is quicker.  Jeweler’s rouge on a cotton wheel works OK, too.  Glass windows are trickier.  I haven't had need to polish many glass windows but if I do I'll see what the neighborhood auto parts store has for polishing windshields and headlights.  

Cracked plastic windows:  If the window is cracked all the way across it must be replaced.  But sometimes only a corner is cracked off due to overtightening of a screw.  If you still have both pieces, spread a little plastic model glue on the broken edge of the larger piece with a needle.  Barely cover the length of the edge--too little glue is better than too much.  Slide the two pieces together on a sheet of waxed paper and hold them until the glue sets--30 seconds or so.  The repair will always be visible but it will be a lot better than a missing corner.

Replacing windows: Making a new glass window is difficult.  If at all possible, buy a replacement window.  If you can’t find an exact replacement, buy an oversize window and sand it down to size.  There is enough adjustment in slide rule cursors so that precise centering and verticality of the hairline is not critical.  Hand sanding with ordinary, cheap woodworker’s sandpaper--150 or 200 grit--will bring a glass to size in a couple of hours.  For a neat finish on the edges, wet sand with silicon carbide paper and progressively finer grits.  The ultimate tool is a 3-inch belt sander with silicon carbide belts ranging from 200 to 800 grit.

I have manufactured ten or twelve glass windows from scratch.  They were acts of desperation.  The first problem was finding glass of appropriate thickness.  (Window glass is much too thick.)  I found 1mm thick microscope slides in my college bookstore; I found other suitable glass in old picture frames and salvaged from glass-fronted furniture.  After cutting and sanding the glass to size I scribed the hairlines with a fine-tipped carbide scribe.  Carbide and diamond scribes both chip glass more than they cut it but you can get a finer point on a carbide scribe.  To minimize chipping I use very little pressure on the scribe--hardly more than the weight of the scribe itself.  A gritty sound indicates I'm pushing too hard.  I take about ten strokes in one direction and then ten strokes in the opposite direction and repeat for a total of about 100 strokes.

Acid etching of hairlines:  Hobby shops sell glass etching paste but it does no more than frost the glass.  I once considered etching hairlines with liquid hydrofluoric acid and purchased all the supplies and equipment I would need.  However I have no experience with acid and the danger to my skin and lungs was more than I cared to risk.  The State send a recycling official to pick up the unopened bottle of acid.    

Most trophy shops can do laser etching.  I have tried it only once.  The resulting hairline was too wide.  That may have been due to my poor instructions or operator error; I'll try again.  The shop charged me about $20.   

In contrast to glass, replacement plastic windows are easy.  One millimeter sheet plastic is readily available from hardware and plastic supply stores.  It can be cut with ordinary woodworking tools and the edges polished with plastic polishes or jeweler’s rouge.  The hairline can be scribed with a straight edge and anything that has a sharp point.

Replacement springs: Small Parts Inc. ( sells 6 x 1/4-inch lengths of spring steel in several thicknesses.  0.010 inches is about right for most closed body rules; 0.007 for duplex rules.  Grind it to width before cutting it to length.  If you have a broken steel measuring tape (the kind that retracts itself into a case) a piece cut to length and width works fine.  Erase the inch marks and painted surface with steel wool.

Re-attaching springs:  Riveted springs can be glued back in place with extra strength epoxy glue.  File off the remaining stub of the rivet and file both mating surfaces clean.  Put a single small drop of glue on the spring and hold it in place until the glue sets.  Extra strength epoxy takes 24 hours to set so I use a jig to hold the pieces in alignment.

Some cursors have springs that were merely crimped in place.  Assuming that there is enough of the spring left to use, you need to remove the temper from an end of the spring, replace the spring on the cursor and crimp it tight.  To remove spring temper, hold the spring tightly with a pliers about ¼” from the end and press the end of the spring against a red hot element on the top of your electric kitchen range.  The pliers are important; they keep the main part of the spring cool.  If you are lucky, the end of the spring will lose its springiness and will hold whatever shape it is bent into.

Rust: Rusted metal can be re-plated.  Check your yellow pages under “Plating.”  I was quoted a price of $5 per cursor frame with a minimum order of $50 in 2000.  No doubt that would produce a superior product but I just take the rust off with steel wool and then use metal polish over the spot to inhibit more rust.

Missing screws: The only way to get exact replacement screws is from salvage of another rule.  The Slide Rule Trading Co. tries to stock screws salvaged from K&E, Post and Hemmi rules.  If exact head match is not important, you can find correctly threaded screws at large hardware stores.  I believe cursor thread sizes are 1-72 for K&E; M1.4 for Post & Hemmi; M2 for Relay/Ricoh.  My friend, John Spivey, says that Hemmi/Post stator screws are M2.5 which, for slide rule purposes, is interchangeable with 3-56.  

Loose screws: Sometimes threaded holes in plastic become so worn that screws will not hold.  Tuck a thin thread in the hole before you put the screw back in.  The end of the thread will be hidden under the metal cursor frame.

Overly tight screws:  Screws on K&E cursors after about 1960 screw directly into the plastic of the cursor bar; there is no threaded brass insert in the bar.  Such screws often require a lot of torque which can lead to damaging the screw head when your screwdriver slips.  A small spot of lubricant applied to the threaded hole with a toothpick solves this problem.  I used to use any liquid at hand--plastic polish or water, typically--but I now keep a small can of light machine oil with my tools for this purpose.  Apply the oil with a toothpick, oil drops directly from the can are too large.

Cursor bars that don't contain threaded metal inserts require extra care to avoid "cross-threading."

Body Repairs

Sticky slides:  Aside from warped rules, I have never had a rule which did not slide properly after it was thoroughly cleaned.  Thorough cleaning includes scraping all surfaces of the tongue and groove and polishing with steel wool.  I have never found talcum powder, powdered graphite, silicone lubricant, Vaseline, or white grease anything but counter-productive on wood and bamboo rules.  However, K&E recommends lubrication with talcum powder on their wooden rules and Pickett gives explicit instructions for their metal rules: "The metal edges of your slide rule will require lubrication from time to time.  To lubricate, put a little while petroleum jelly (White Vaseline) on the edges and move the slide back and forth.  Do not use ordinary oil as it may discolor rule surfaces."

Warped rules:  Slide rules are not supposed to warp but sometimes they do.  If you can possibly put up with the warp, do so.  If you must correct a warped rule, here’s what worked for me:  Remove the cursor but otherwise leave the rule together.  Using wood blocks and C clamps, clamp the rule to a sturdy piece of wood—such as a 2 x 4.  Slide wedges under the rule until the warp is reversed—that is, the rule has about the same amount of twist but in the opposite direction.  Wrap the rule, clamps and board in an electric heating pad.  Set the pad temperature to 120 degrees or so and check the warp every week or so.  It took about two months to decrease an obviously warped rule to a subtly warped one that I could live with.  I have tried this only on a wood rule but I believe it would work on plastic and bamboo.

Loose celluloid veneer:  Ordinary carpenter’s white glue (e.g., Elmer’s Wood Glue) is perfect for gluing down loose celluloid veneer.  It has to be clamped overnight—tight but not so tight that you squeeze all the glue out of the joint.  Wipe up the squeezed out glue with a damp sponge before it has a chance to set up.

Missing celluloid veneer:  Sometimes the celluloid veneer isn’t just loose, a whole piece is broken off and missing.  You can make a decent replacement with patching material that is sold for filling chips in porcelain sinks.  However, in my opinion, this crosses the line from “repairing” to “refinishing;” I wouldn’t do it.

Engraved graffiti:  What can you do if the previous owner scratched his initials into the rule?   Not much if the graffiti is on the metal brackets.  In theory, you could grind graffiti off of solid metal brackets but there certainly isn’t anything that can be done if the brackets are plated.  Graffiti in the celluloid veneer is easier; it can be sanded away with emery cloth.  Be sure to protect the rest of the rule with masking tape.

Loose slide:  Sometimes no adjustment of the stators will provide enough friction to keep the slide in place.  The problem often seems to be that time has shrunken the wooden parts of the rule but not the metal brackets and the stators no longer bear firmly against the slide.  Relay/Ricoh rules seem particularly prone to this problem.  You need to enlarge the adjusting holes in the movable stator so that it can be moved closer to the stationary stator.  Take the removable stator completely out of the rule and elongate the holes in each end with a file.  A thin rat-tailed wood rasp would be perfect but I have not seen one that is thin enough.  The best one can do is metal worker’s “Pattern-making” or “needle” files.  They work--but very slowly on wood or bamboo.

Slide Rule Cases

Cleaning and waxing: I recommend “Lexol” leather cleaner in preference to old-fashioned saddle soap.  It is a liquid cleaner that is applied with a damp sponge.  I do not clean a case until I have finished all repairs such as re-stitching and re-dying.  The cleaning process blends the repaired colors and textures together.

After cleaning, I wax all my leather cases with shoe polish if I can match the color.  (I believe shoe polish contains some leather preservatives.)   I use Johnson’s Paste Wax if I can’t match the color.

Loose stitching:  Do not take a case with loose stitching to the shoe repair shop; it must be re-sewn by hand.  You want to use the original holes, not punch new ones. You can find a good match for the thread color at any fabric shop; they stock thousands of colors.  When finished sewing, lock the new stitches in place by running a pin coated with wet Instant Krazy Glue though the holes.

Graffiti:  Graffiti on rough leather (e.g., inside the flap) can be sanded away.  Gold leaf can be removed from smooth leather with “deglazer.” (See your local leather store.)  Ball point ink can also be removed with deglazer but unless the ink is very fresh, deglazer usually removes an unacceptable amount of color from the leather, too.  Impressions in smooth leather can be removed by dampening the leather and pounding it with a smooth hammer.  To do this on the body of a slide rule case you must fill the case with a hardwood last.  Few cases are worth the effort.

Discolorations:  Cases often come with discolored spots or you add some while trying to remove graffiti.    You can eliminate these by re-dying the case darker than its original color.  However, changing the case color crosses the line into “refinishing.”  Touch up dying to match the original case color is acceptable but tricky.  Except for black cases, my success rate has been so low that I avoid it if possible.  If you must touch up spots on a leather case, do so before you do any cleaning on the case at all.  Clean and wax the case after the dye dries.

For black cases, “Fiebings Leather Balm” is great.  It is a combination of black dye and wax.  Be sure to buy a dauber from the leather store.  Mask off any lettering, etc., you don’t want to become completely black.

So far as I know, there is no cure for the spots where sticky tape has been removed from smooth leather.  (If you ever want to ruin a leather case, stick a piece of labeling tape on it.)

Dry leather: There are many products for treating dry, crumbling leather.  My favorite is “Hide Rejuvenator.”  It looks and feels like white grease but I’m told it is really crude lanolin.  It leaves the leather greasy and discolored for months until it has completely soaked in and dispersed.  Even then it doesn’t solve the problem; it just stops further deterioration.

Deformed cases:  You can completely reshape a case by dampening it and placing it over a form.   For smaller reformations, use steam.  There are a couple of brands of “steamers” on the market that will allow you to deliver steam to the particular area that needs softening.  An old-fashioned teapot works, too.  I have not tried reshaping a lined slide rule case but if I ever do, I’ll try to keep the lining in contact with the shell as it dries by inflating a balloon inside the case.

Rips and tears:  A shoe repair shop can glue a thin leather backing to mend tears in leather.  Don’t let them sew it on.  This is about the only thing you can do for a case whose flap is ripped off.  Unlike most of the other suggestions here, this is an obvious repair and the case will always be “repaired.”

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