Why baby/mineral oil?
Wet or Dry hone
Some people use oil stones and wet/dry abrasives without oil. They argue that you shouldn't use a lubricant when you are trying to scratch the tool. Sounds reasonable, but it is not.
From Handbook For The Artisan, Mechanic, And Engineer 1870:
"The oilstone should be moistened with good clean oil not disposed to dry; otherwise, it becomes thick, like glue or varnish, and entirely prevents the action of the stone upon the tool."
In fact, the oil is used to float the filings up off the surface of the abrasive. The blade then pushes the floating filings away from the part of the abrasive in use. Think of the oil as a cutting fluid. [According to Metallographic Polishing by Samuels, cutting or abrasion fluids have no effect on the rate of abrasion, but do flush the abrasion debris away from the working surface.]
The idea that dry honing is somehow better than wet honing has been popularized by a book called The Razor Edge, which is discussed below. The author claimed better results using a dry fine silicon carbide stone for final honing of knives used in industrial meat cutting shops. A fine silicon carbide stone is about 240 grit. If you hone with such a coarse stone then you should expect unusual results. Unless you think that 240 grit is a suitable final honing grit, don't follow his advice.
In case there is any doubt about my recommendation here: Never hone (or grind) with a dry abrasive!
Of course, powered grinding is a bit different because the belt or wheel throws most of the swarf off the abrasive (and into the air and eventually into your lungs).
I use baby oil - which is a light mineral oil. Get the fragrance free variety. You can buy mineral oil, but it is often of a thicker consistency, which is almost useless in this application. The pharmacy of any large department store will have baby oil. The generic version is just fine.
Baby oil is also a good cutting fluid if you use wet/dry abrasive paper to remove rust/grime from hand saws, or other steel or cast iron tools.
Honing with a liquid present
Filings remain in suspension in the liquid, allowing the tool to sweep them out of the
region of the abrasive being used. This keeps the abrasive clean, allowing for quickest honing.
When you pull the blade toward you, you can see a wake in the liquid. This wake should be uniform
across the blade. Any setup problems, such as not having the blade square in the jig, are immediately evident.
A non-volatile liquid is best
Volatile oils that dry out are generally more harmful to your health.
Mineral oil does not give off harmful vapour.
Water is used as a cutting fluid in some stones, but it dries even over short sessions,
does not seem to lift filings as well as mineral oil, and remains on the tool after honing.
A quick wipe of the oil off the abrasive sheets with a cloth removes most of the filings,
readying the abrasives for the next session.
Any filings that remain are held by the oil and will not drop off during storage,
generally keeping the storage area free of filings.
A light coating of oil on your hands before you start will make it easier to clean your hands afterwards. (A tip from Blake Ashley on the oldtools list.)
The tool has a light coating of non-drying oil that helps protect it against rust.
Some people think that any petroleum by-product is dangerous. The Wikipedia article on mineral oil disagrees.
People use and swear by other oils related to baby oil. Here is some information from Material Safety Data Sheets for various oils.
In case of contact, immediately flush skin with plenty of water. Cover the irritated skin with an emollient. Remove contaminated
clothing and shoes. Wash clothing before reuse. Thoroughly clean shoes before reuse. Get medical attention.
The substance is toxic to the nervous system. The substance may be toxic to blood, kidneys, liver, central nervous system
(CNS). Repeated or prolonged exposure to the substance can produce target organs damage. Repeated exposure to a highly
toxic material may produce general deterioration of health by an accumulation in one or many human organs.
Chemical gloves should be worn to prevent repeated contact. If potential for significant exposure to
liquid exists, use full protective clothing and chemical boots.
NIOSH-approved organic vapor air-purifying respirator, self contained breathing apparatus, or air supplied respirators dependent on concentration
Skin contact: No effects expected.
An alternative that seems comparable is to use a magnet! Put a magnet on the blade and the filings will stick to the blade rather than remain on the abrasive. Periodically you remove the magnet and the filings fall off.
I have some experience with this effect - something that just happened during the Norton 3X testing. For some reason, grinding on the very coarse Norton 3X papers caused the blades (all blades used in the test) to become magnetized! All the filings adhered to the blade, so did not clog the abrasive. They had to be wiped off the blades, since the blades appeared to be permanently magnetized.
Unfortunately, the bits of abrasive are not magnetic and do remain in the abrasive. The abrasive becomes clogged and has to be cleaned somehow.
All in all, baby oil is a much better choice. Once the honing/grinding session is over, a wipe (or a blotting up with coarser grits) with a rag and the abrasive is clean, with no dust (filings, broken abrasive) in the air.
The Razor Edge
A sharpening book written in 1985 called The Razor Edge reported comparison tests of knives sharpened on oil-stones with and without oil. Those tests were said to clearly favour sharpening without oil.
I have not been able to find any evidence that these claims are true. If you look at my micrographs of plane blade edges, you will find no evidence that the filings floating in the oil damage the edge. In fact, if you look at pictures of the sharpening station you can clearly see the swarf has been for the most part swept out of the active area of the abrasive. The same is true for swarf on the bench stones.
I suspect that the effect they encountered may be a result of using too coarse a final honing abrasive. It is not clear what grit they used for honing - the book only mentions medium and fine abrasives. If their oil stones were Norton crystolon stones, then the fine stone was 240 grit, or about 54 microns. Their final abrasive was almost 4 times are coarse as my first abrasive. When used with oil, the broken abrasive is swept out of the honing area so the abrasive is always 240 grit. Perhaps when used without oil, the broken abrasive remains in the honing region creating a swarf, much as with Japanese water stones. This swarf may have a smaller actual grit size (larger grit number) than the stone itself, resulting in a honed surface with smaller scratches. That is what the proponents of Japanese water stones claim.
In both cases (too coarse an oil stone used dry, Japanese water stones), relying on the breakdown of the abrasive to create a new finer abrasive seems more magic than science. If you don't hone long enough - how long is long enough? - you don't get the effect you need. Some people may get good results, some using almost the same technique may get poor results.
With microbevels, these 3M abrasives, and a honing oil, you get consistent results - no magic.
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Can you dub the edge with abrasive papers?
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What is dubbing?
As you push the plane blade forward on the abrasive, imagine the abrasive rising up slightly in front of the blade edge. The blade is not being honed by a flat abrasive -- it is being honed by a curved abrasive. The result is a bevel that is not flat. It has the effect of increasing the included angle right at the edge. This is referred to a dubbing.
This effect is common on leather strops. There you have a soft surface which compresses under the edge, rising as soon as the edge passes (you pull in this case of course). A leather stop will always dub the edge slightly (another good reason not to strop). The harder the strop the less the effect - buffing compound on hard maple does not dub.
You get the same effect with buffing wheels - even with hard felt buffing wheels. The harder the wheel the less the effect.
This use of the word dub, to round over, is not in any online dictionary I looked at.
So, will abrasive paper dub?
Yes, it will. If you do not use glued down abrasive, the abrasive could easily ride up in front of the blade and dub the edge. If you press really hard, the effect will be greater. If the abrasive backing is flexible or weak, allowing the abrasive to bunch in front of the blade, you can get dubbing. If you use the abrasive dry or if you allow too much swarf (especially the bits of the wire edge that break off when you switch from side to side) to build up, you could get an abrasive effect from them which would dub the blade.
Does the PSA backed 3M microfinishing paper dub?
Not if you
Use the paper wet (with a light non-drying oil like baby oil),
don't push really hard (you don't have too),
keep the paper clean (wipe down after each blade, or more often if the swarf is bad).
How big should the glass sheet be?
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How big should the glass be?
Update 2006 I have switched from putting 3 different abrasives on a single sheet of glass to using multiple sheets of glass. My current setup uses 6" x 16" x 5mm glass, glued to 1/4" Masonite. Look here for a picture and here for a discussion.
In the past I use one large sheet of glass. The result is not as portable, but is still an option. Get a larger piece than you need and you will have lots of room for parts of the three sheets of abrasive. A piece 15" wide by 14" long should do pretty well. The glass need not be really thick, since the rigidity is provided by the (plywood, Masonite, MDF) backing board, and of course the bench on which that sits. If you have to bear down on the iron to get it to sharpen, it is time to freshen your abrasive paper.
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