GEC Northfield UN-X-LD knife with 1095 carbon steel blades. Image courtesy of Collector Knives.

Knives with carbon steel (as opposed to stainless steel) blades are a significant component of the current knife scene. To illustrate, about 12 percent of the knives cataloged on the website of blade HQ, one of the major Internet marketers of knives, have carbon steel blades. Notably, this includes knives with D2, 1095, A2, O1 and otherwise unspecified "high carbon" steel blades. Some knife makers, such as Great Eastern Cutlery, use carbon steel in almost all of the knives they produce.

The generally cited virtues of carbon steel blades include ease of sharpening and ability to take a very keen edge. Actually, depending on its specific composition, carbon steel has better edge retention than might be expected, given how easy it is to sharpen. In addition, carbon steel is one of the toughest (i.e., most damage resistant) types of blade material.

Unfortunately, the biggest downfall of carbon steel is that it has little rust/corrosion resistance. Carbon steel blades have some desirable performance characteristics, but anyone who owns and uses them needs to be aware of a few simple things concerning how to care for them, in order to cope with their susceptibility to rusting.

This article is an attempt to provide key tips on carbon steel blade care, focusing on preventing rust damage. The issue of patinas on carbon steel blades will also be addressed. Finally, an addendum is included with some thoughts on D2 steel, a special case among the steels that are the focus of this article.

Priority 1: Prevent rust

I do not believe I need to provide a technical definition of rust. Suffice to say it is the formation of iron oxide on a steel surface. Every knife user recognizes the ugly spots and blotches that can form, remarkably quickly, on knifes that are not maintained properly. The results of rusting include pitting of the blade surfaces, deterioration of the cutting edge and, in folding knives, damage to the tang, blade pivot, back springs, or lock mechanism. Not good.

The prime rusting agents of knives in general, and those with carbon steel blades in particular are (1) moisture and (2) corrosive agents. As defined in Wikipedia, a corrosive agent is: "Any chemical that will dissolve the structure of an object [including] acids, oxidizers, or bases.

Therefore, the key to preventing rust is to remove moisture from the blade as soon as possible, along with any residue from cleaning a game animal or fish, cutting live vegetation, or food preparation. Cleaning off such residue (hereafter simply "gunk") is important, because it will hold moisture against the steel, plus the gunk itself may be corrosive.

The cleaning tips that follow are written with carbon steel knives in mind, but they also apply to stainless steel blades. The various stainless steel alloys used in knife blades will eventually rust if cleaning after use is neglected.

The first line of defense against rust on any knife is cleaning it after use. This is not rocket science. For instance, a knife used to field dress a game animal should be wiped dry as soon as possible. Wipe off the blade (and handle, if needed) to remove moisture and any gunk that may be adhering to the knife.

This should be done to fixed blade knives before they are returned to the sheath and to folding knives before the blade is closed. This is to prevent anything that might promote rusting from getting into the sheath, or inside a folding knife's handle.

At earliest convenience, give the knife a bird bath to remove any tenacious gunk that resisted the initial wipe down. Use a damp paper towel or cloth, with just a touch (literally) of general purpose cleaner (I like Simple Green), or dish washing detergent to thoroughly clean the blade. Follow by wiping with a paper towel or cloth that is dampened with water. Then rub the knife dry and leave it to air for a while; out of the sheath for fixed blade knives and/or blade(s) open for folders.

Whether a knife needs a bath is a judgment call. If I only used it to slice cheese and sausage for a field lunch, then I would just wipe it off thoroughly. After field dressing a deer, though, I would give it a bath.

An acceptable alternative to using water and detergent for cleaning blades is to use rubbing alcohol. I do not advocate using alcohol, or any chemical solvent, to clean handles or folding knife mechanisms. Many folding knives, especially slip joints, have dyed bone or wood handle covers. If a solvent is used in the handle cavity, it will likely bleed onto the covers and will mess up the dye or wood finish. This would not be good.

What about putting a protectant of some sort on the blade? If the knife is going to be carried and used again right away, that is not necessary, although it cannot hurt anything.

If the knife is going into storage for awhile, a very light treatment of protectant is a good idea. My first choice is drugstore mineral oil, with a light vegetable oil (such as canola or olive oil) being a close second.

No need to drown the blade. A very light coating of protectant, using a square of toilet paper as an applicator, is sufficient. Wipe it on so lightly that you can hardly tell it is there.

Never put a cleaned and dry knife into a wet sheath or carry pouch. This should be obvious, but I have to say it. Any sheath, whether leather, ballistic cloth, or plastic (Kydex, etc.) that gets wet must be allowed to dry completely before using it to carry the knife again. Also, long term storage of knives in leather sheaths is a bad idea. Storage in synthetic sheaths is okay, as long as the sheath is clean and dry inside.

Folding knife mechanisms may require special attention

Additional cleaning measures are in order when gunk gets into the blade channel, spring, or lock mechanism of a folding knife. The low-tech tool for cleaning out small recesses in knife handles is wooden toothpicks. Use the heavy duty, round or square ones.

To loosen the gunk, put a tablespoon of water into a small cup, stir in a drop or two of general purpose cleaner or detergent, dip a toothpick and dig and scrape at the gunk until it loosens. In addition, dig out any gunk that has accumulated in folding knife blade nail nicks or thumb holes, or around the base of thumb studs. The toothpick will not hold much of the cleaner, so dip it several times in the course of a cleaning.

After working the gunk loose, flush it out of the handle with water under a running hot water tap, if necessary. (In the field, use a small squirt bottle filled with water.) The idea is to flush away all the gunk.

Shake as much water as possible out of the handle and wipe the knife dry. Then, grab a hair dryer and blow dry the innards of the handle; about a minute, on low heat, should do it.

Finally, lubricate the blade pivot/tang area, back spring anchor pin spots and lock mechanism (if equipped). Do this while the knife is still warm from the dryer, so the lubricant will flow and penetrate better. Use a low viscosity lubricant formulated for knife mechanisms, or fishing reel oil (formulated for revolving spool casting reels). A repurposed eye drop bottle, filled with mineral oil or light vegetable oil, also works.

Do not overdo the lubrication. One or two drops in the pivot/tang and locking mechanisms, and one drop on other spring pivot or anchor points is sufficient. Lay the knife aside on a folded paper towel overnight, then wipe off any excess lubricant. Check that the blade cycles smoothly and put it back into service.

Commercial cleaners/lubricants/protectants

Frankly, I am not sold on most of the commercial products that are specifically touted for cleaning, lubricating, or protecting knives. Generally, they do not offer any obvious performance advantages over the common products that I have already mentioned and they are usually sold at greatly marked-up prices. If you have had good experience with a type or brand of such products, this is fine. Feel free to use it. Otherwise, go with sparing use of the cleaners, lubricants and protectants that you have on hand.

Two additional notes on this subject are in order. First, what about caring for wooden or leather knife handles? There are many products and strategies, but for cleaning, polishing and protecting wooden or leather handles I have not found anything that works better than spray-and-wipe furniture polish (Pledge, or similar). Spray a bit onto a cloth or paper towel, thoroughly wipe it onto the handle, and it is all good.

What about using gun maintenance products on knives? I do not go there, mainly because these products may contain chemicals that might taint any food products the knife is used to prepare. Some gun maintenance products contain toxins that it would be undesirable to ingest, even in small amounts. Again, it is user's choice, so go for it if you are comfortable with and get good results from using gun maintenance products on knives.

Patina

When I was growing up, everyone and their cousins had carbon steel knives. We used them hard, kept them clean and dry enough to avoid rusting and took it for granted the blades would tarnish over time. ("Tarnish" is an old term for what is nowadays called "patina" in knifespeak.)

Patina is the discoloration that occurs naturally on the surface of carbon steel. It is the manifestation of a slow, benign oxidation of the steel. Put another way, patina on a knife blade is a form of rust, but not the fast forming, destructive kind that happens when a knife is left dirty and wet. Patina is evidence of a knife that has been carried, used and properly cared for along the way. Consider it a badge of honor.

Here is an image of a natural patina on a blade. I would expect the color contrasts to mute and blend over time. (Quick quiz: Why is there no patina on the tang?)

Patina on a carbon steel blade. Image courtesy of thetruthaboutknives.com.

Besides its authenticity, a naturally formed patina has a practical benefit. Great Eastern Cutlery (GEC), which uses 1095 carbon steel almost exclusively in their knives, explains as follows:

"Over time you may notice your blade changing color. This is a natural process during which the steel gains a patina from normal use and age. A patina not only adds character to your knife, but helps protect your carbon steel from rust which might cause pits."

Formation of a natural patina takes time. For instance, I have a two blade GEC Northfield pocket knife (image at top of article) that I have carried and used for almost two years. The patina on the blades is developing nicely, but does not yet cover the whole surface of either blade.

Natural patinas typically begin to form along both sides of the blade spine and slowly creep downward toward the cutting edge. Why it happens this way is a mystery.

This leads to the topic of forcing a patina. As GEC notes, "Many people find a satisfaction in watching how their patina develops over time while others 'force' a patina in order to gain a more uniform coloration."

I took the trouble to study a number of articles and videos on forcing patinas, thinking that I might write a summary article on the subject. I have scrapped the idea of an article about patina forcing and instead will just note a few things about it.

First, the most popular method for forcing patinas involves using vinegar, lemon juice, or lime juice, because these are mildly acidic. Immersion of a carbon steel blade in any of these will enable the acid to form a uniform patina on the surface of the blade.

Depending on how dark a patina is desired, this may take several hours if the vinegar or juice is used at room temperature. Some aggressive individuals advocate using vinegar heated to boiling, which can reduce the blade immersion time to a few minutes.

When the desired shade of patina has been achieved, the process must be stopped by rinsing and drying the blade. (This is critical if the hot vinegar method is used.) The purpose and result is to get a uniform patina, anywhere from light gray to charcoal gray in shade.

At the other extreme, there is what I call the condiment patina, since the method uses mustard, ketchup, or mayonnaise, which are all mildly acidic. The condiment is dabbed lightly onto the blade and left to set for at least two or three hours. Then, the blade is cleaned and the results assessed.

The patina will be most pronounced wherever the condiment was the thinnest and at the edges of any bare areas on the blade. The effect is an irregular patina pattern, which is developed fully by multiple applications of the condiment (generally at least three). The visual effect is very different from either a natural patina, or one forced using the liquid immersion approach.

Call me old school, but the reason I dropped the idea of a feature article on forced patinas is I have decided that, aesthetically, I do not like them. If I want a uniform colored coating on a knife blade, I can buy any of a number of knives which already have some sort of baked-on or chemically applied coating on the blade. (A whole other subject, for another time.)

As for the condiment patina, the splotchy affect this technique typically achieves, based on images I have seen, does not grab me at all. I am content to let the patinas on my knives form naturally.

There is one thing I must mention about knives with factory coated, carbon steel blades. Do not assume that owning such a knife gives you a pass on the essential rust preventing maintenance measures. The ground bevels of such knives generally are not coated, which means that these areas are exposed to potential rusting, just like an uncoated blade. Plus, if the coating on the flats of the blade gets scratched or worn off, those areas are also vulnerable to rusting.

Or you can polish

Some knife users may prefer to keep their carbon steel blades polished, rather than letting them develop a patina. Doing this is simple, really. Use a readily available metal polish (Flitz, or similar) to polish the blade. Patina develops slowly, so a knife that is being used can be kept bright by polishing the blade two or three times per year. A knife that is in storage should be inspected and touched up roughly once a year. After a blade is polished, it should be treated with a thin coat of mineral oil, or whatever protectant one prefers. That is all that need be done to keep the knife looking nearly like new.

Incidentally, a partially or fully formed patina can be polished off. It takes some time and patience, but it is a straightforward task if you decide you do not like the patina on your knife.

Conclusion

As I said, basic maintenance of a knife is not rocket science. Moreover, it is not as time consuming as it might seem.

The immediately-after-use wipe down I described as the first step in essential maintenance can be done in thirty seconds, or less. A routine "bird bath" generally takes less than five minutes. A deep-dive cleaning of a folding knife handle usually can be accomplished in no more than ten or fifteen minutes, including power drying and lubing the pivot, springs and lock.

Cleaning out the handle generally needs to be done infrequently, unless you habitually drop your knife into mud holes, or some such foolishness. To me, these are very reasonable expenditures of time and effort to keep a knife rust free and in good working order.

Addendum: D2 as a transitional steel

I mentioned at the top of the article that D2, 1095, A2, O1 and miscellaneous other high carbon steels are popular in the knife market. In general, so-called high carbon steels are those that have a carbon content of 0.8 percent or more (by weight) in the alloy mix. I noted the desirable properties of these steels for use as knife blades, but also noted that they generally have the undesirable property of having little resistance to rusting.

What I did not stress earlier is that the high carbon steel alloy known as D2 is the outlier among the most popular carbon knife steels, in the sense that it exhibits rusting resistance to the point that it is frequently called a "semi-stainless" steel. The key to formulating a steel alloy that is rust resistant is adding chromium to the alloy mixture. The amount of chromium that is needed for an alloy to be classified as a stainless steel (i.e., highly corrosion resistant) is generally cited as being 12-13 percent, by weight. (Depending on the source, as they do not all agree.) D2 steel alloy nominally includes 11.5 percent chromium, just a bit short of the stainless steel threshold.

By comparison, 1095 steel contains no chromium, while A2 and O1 steels contain chromium in nominal amounts of 5.25 percent and 0.4-0.6 percent, respectively. Clearly, these three steels are not in the same class with D2 when it comes to chromium content and, therefore, resistance to rusting.

(The small amounts of chromium in A2 and O1 are there for an entirely different purpose than to give the steel rust resistance, which is not relevant to this discussion.)

I have four knives with D2 blades in use. These have shown no signs of wanting to rust under my routine maintenance regime. The blades have lost some of their shine, but without the distinct color changes associated with development of patinas. Given that the steel seems disinclined to raise a good patina, I am going with an annual polishing of these blades. From my experience with these knives, I conclude that the characterization of D2 as being semi-stainless is appropriate.

D2 alloy, with a nominal carbon content of 1.5 percent (the highest among the steel alloys discussed here), shares most of the desirable performance properties of other high carbon steels, especially ability to take a keen edge and good edge retention.

However, D2 has been criticized as being relatively hard to sharpen and is also reputed to be prone to chipping of the cutting edge. By contrast, the more normal carbon steels (such as 1095) are valued for being quite easy to sharpen and having exceptional cutting edge integrity; i.e., they are not prone to chipping or other damage to the cutting edge.

I have had no problems with cutting edge integrity on my D2 knives, either during normal use, or when sharpening. However, I have not torture tested these knives, so I cannot argue that reports of chipping or other cutting edge problems are total bunk.

Despite the hard to sharpen knock, I have never been stymied when sharpening D2. To me, it behaves much like CPM-S30V and CPM-S35VN under sharpening. The key to sharpening D2, along with S30V, S35VN and similar premium steels is to use diamond plate or ceramic sharpeners and realize the sharpening job is going to require a bit more time and patience than sharpening more plebeian steels, such as 1095, 440C, etc.

All things considered, I have come to think of D2 as a transitional steel. By this I mean it has most of the desirable performance characteristics of time tested and proven high carbon steels (e.g., 1095), but is much more rust resistant, even if it is not technically a stainless steel. Therefore, D2 bridges the gap between carbon steels that have very little corrosion resistance and common stainless steels, such as those in the 440 series.

The transitional nature of D2 does not stop there. There is a pretty strong consensus among those who study and evaluate blade steels that D2 has edge retention, toughness and ease of sharpening characteristics close to those of the most popular new "super steels," S30V and S35VN being specific cases in point.

The only key performance characteristic in which D2 clearly falls short of S30V, S35VN and similar modern steels is in corrosion resistance. This is not hard to understand, for S30V and S35VN both contain 14 percent chromium in their alloys. D2 has 12 percent chromium and that 2 percent difference is highly significant in terms of corrosion resistance.

Why all this discussion of D2? Because it has quietly become a prominent high carbon steel used in a significant number of production knives. To quantify this, as of December, 2018, the Blade HQ website listed (in rounded numbers) 1700 knives with D2 blades, a number nearly equal to knives listed with 1095, A2 and O1 blades combined. Knives with D2 blades are definitely in play in the marketplace.

D2 is summed up as follows in a knife steel guide on the Blade HQ website: "Good edge retention, toughness and sharpenability, D2 has been a popular knife steel dating back to WWII. Keep your blade oiled, as D2 is prone to rusting and corrosion. This is one of those most ubiquitous steels."