Wood Knife that is 3x Sharper than Steel? Spoiler: No

Several people have sent me links to a recent publication about a process that increased the hardness/strength of basswood. They then demonstrated the success of their processing by making a knife and a nail out of the wood. See here for an example of one of these articles: https://www.cnet.com/news/scientists-create-a-wooden-knife-thats-three-times-sharper-than-steel/

And there has been another resurgence of discussion with a NY Times article: https://www.nytimes.com/2022/04/11/us/hardened-wood-knife-history.html

The journal article was published in the journal Matter which I am not familiar with. I don’t know if it is a high profile journal or not, but I will focus on the contents rather than speculating on the quality of the journal. Here is a link to the article in the journal: https://www.sciencedirect.com/science/article/abs/pii/S2590238521004653 Unfortunately you won’t be able to read it without paying for it.

I am not an expert on wood so it is hard to say exactly how “revolutionary” the wood processing is. Wood is made of cellulose, lignin, and hemicellulose. Hardwoods have a higher proportion of cellulose, the strongest component to wood. One of the main claims of this research is that they used a chemical process to remove lignin, thus leaving a higher proportion of cellulose and strengthening the wood. However, the graphs they showed have a relatively small amount of lignin removed, and the highest strength was not found with the lowest lignin content:

Instead by their own admission the hardness was correlated with the density of the wood, which peaked at the 4 hour treatment. After the chemical process, they did a cold press and hot press to the wood to increase its density. Therefore it is not clear how much of an effect the small reduction in lignin contributed or if it was primarily the increase in density, similar to other wood densification processes.

I did a very short literature search for wood densification processing and easily found many papers on the topic. Here is a big thesis with a lot of references: https://www.diva-portal.org/smash/get/diva2:1347640/FULLTEXT01.pdf From the literature search in this thesis they concluded that hardness of wood is greatly controlled by its density, or for a given wood the degree of densification. None of the literature on wood densification is cited in this paper on “wood that is sharper than steel.” Not a single reference. This is extremely irresponsible, and part of the reason why the internet media is eating it up as a big advancement. If you want people to get excited about your findings, just conveniently ignore all prior similar research. Well, ok, the web media would still report the findings without actually looking at any references. You got me. In the thesis I just linked to, which I do recommend with a lot of good references and experiments, they rank different woods on their potential to be densified and basswood was one of the highest ranked, so that helps explain why it was chosen for this wooden knife example to demonstrate how successful their process is. It gains a lot of strength through densification so it looks impressive on the charts with the huge increase.

Because the hardness is controlled by density, if the hardened wood is soaked in water it swells up to near its original dimensions and loses its density, and therefore its hardness. In the paper they soaked the wood in mineral oil which helped prevent the uptake of water. This is also not an original idea.

Because of the very small reduction in lignin, and the fact that hardness and density are so directly linked, I cannot tell if their process is better than previously reported experiments. Also the type of Brinell hardness test they performed is not the same as in previous literature so they can’t be directly compared. Maybe their process did lead to a superior hardness increase but it’s hard to tell when they don’t compare to any other similar process.

Uses of the Wood in Nails

OK so that’s enough about the wood, let’s get to the practical examples they showed. One was that they made a nail out of the hardened wood. Wood doesn’t rust so this is potentially advantageous when compared with steel. They tested their wooden nails and found them to work under the type of stresses that a steel nail will see. However, I googled wooden nails and found a company that makes some already. They even work in a nail gun which seems impressive: https://www.beck-lignoloc.com/en So again there is no discussion as to how their wood densification process leads to an improvement over currently available wood products.

Use of the Wood in Knives

And now to my area of expertise: knives. They compared their wood to “commercial table knives.” In other words, butter knives. These are of course not designed to be sharp. Here are pictures from the article:

In fact they already found a wood butter knife to compare against that was available at a store, so already there is a lack of novelty to the wooden table knife application. The cross-section images of the knives already show that the hardened wood knives (labeled Type I and Type II) are going to beat the steel and commercial wood knives. The steel knife is intentionally blunted. It is not designed to cut things other than butter.

The method they used to test sharpness is one I am very familiar with: and Edge on Up tester with BESS media. This is a scale with a mounted thread. You press the edge into the thread until it is cut and the maximum force is found on the scale. Here is a YouTube video of (someone else) demonstrating one:

I use this tester for checking all of my knives prior to CATRA testing so I have used it a bunch. A knife above about 800 grams or so is completely, absolutely dull and you would have to work very hard to manage to cut yourself with it. Rub your fingers on there hard and you will be completely safe. Above that point instead of cutting the BESS thread, the thread tends to snap under the force instead. And the scatter is very high. I do not find the tester useful for levels beyond that. All of their measured values were 1500g (1.5 kg) or higher. Ignore the sliding measurements as the tester is not really designed to work that way:

So their hardened wood knife showed superior sharpness because they made it sharper. They didn’t do comparisons of how sharp they could get the different materials. They didn’t even test to see how sharp they could get the commercial wood table knife as far as I can tell. Obviously the steel can be sharper. Razor blades are regularly under 50g. So the internet media reports of “wood that is three times sharper than steel” are obviously false and just lazy reporting.

Summary

So did they achieving something new with their process? Can’t tell, they didn’t compare to anything else or even cite any other studies with a similar process. Other wood densification processes have already been reported on and tested but these authors pretended they don’t exist. Are the knives or nails revolutionary? No, there are already butter knives and nails made out of wood. Did they successfully get a bunch of online media outlets to report on their research because they had a good hook on a press release? Yes.