Bonsai tool materials – carbon or stainless steel?

A fellow bonsai enthusiast at my bonsai club asked for advice from club members on their choice of tool materials, which prompted me to look into the physical differences between the materials on offer.

The bonsai tools most people have include a range of branch, root and knob cutters, pruning shears/secateurs, scissors and pruning saws. A quick surf of bonsai stores online show that these tend to be made from steel of one form or another. Steel is an alloy made from iron and carbon. Its properties are adjusted by steelmakers by changing the level of carbon in the steel, by heat treating it or by adding other materials to the alloy such as nickel or manganese.ref Many different types of steel are made with different combinations of these factors and the manufacture of steel tools can be an artisanal enterprise, with specific alloys and processes resulting in tools with different properties.

The Japanese are known for their hard steel called tamahagane which was used for forging Samurai swordsref1. Iron-rich sand from Shimane was smelted with charcoal in a furnace, resulting in a high carbon steel which was then folded over and over to create a very strong, sharp sword.ref Japan still has a massive steel industry and they are 3rd largest producer in the world after India and China, they have a reputation for ‘higher quality’ or at least higher carbon steels than other countries.ref

The properties that are important for bonsai cutting tools include (1) strength, (2) the ability to hold an edge (stay sharp), (3) resisting deformation under stress (ie. not bending), (4) longevity, (5) rust resistance and (6) price to manufacture/buy. Hardness is the term used by steel manufacturers to encompass 1-3 of the above properties. In general a harder material is stronger, less able to be deformed by stress and more resistant to abrasion.ref Ease of sharpening is inversely associated with hardness, as resistance to deformation or abrasion means resistance to sharpening. The downside of hardness is that this type of steel may be more brittle, as it would break or snap instead of deforming under stress. Knives made from very hard steel can chip or break more easily.

So for a bonsai tool which is strong, holds its edge and stays true, you are looking for a high hardness number (in the knife industry the Rockwell scale of steel hardness is used – HRC). However the harder it gets, the more difficult it will be to sharpen.

The types of steel you might encounter when buying a bonsai tool include:

  • High carbon steel – higher carbon content is the main way that steel is made harder. Exactly how hard depends on how much carbon has been added. High carbon is usually 0.6% carbon content or more and a HRC rating of >60.
  • White steel #1 is 63+ HRC containing 1.25-1.35% carbonref (officially ‘high carbon’ steel) also known as Shirogami in Japanref
  • White steel #2 61-62 HRC, with 1.05-1.15% carbonref (officially ‘high carbon’ steel)
  • White steel #3 60 HRC with 0.80-0.90% carbonref (officially ‘high carbon’ steel)
  • Blue steel #1 & #2 also known as Aogami in Japan, are white steel #1 & #2 with added chrome and tungsten, and HRC 58-60 & 63-65 respectivelyref The chrome gives a stainless steel component with rust resistance.
  • Super blue steel is blue steel with added molybdenum and vanadium, HRC 63-64ref
  • ‘Powdered’ steel can be extremely hard at 64-68 HRC – a product known as ZDP-189 contains 3% carbon and 20% chromium so is extremely hard and rust resistance. But I can’t find any bonsai tools which use ZDP-189, it’s mainly used for knives.
  • Medium carbon steel has between 0.3-0.6% carbonref and has HRC <60
  • Black carbon’ steel isn’t a type of steel, it’s a brand name used by Chinese manufacturer Ryuga or a description of a colour (eg. a black coloured carbon steel tool)
  • Low carbon steel has 0.3% carbon or lessref and you ideally don’t want tools made of this
  • Stainless steel is a steel with at least 11% chromium content, which reduces its vulnerability to rust. It’s not necessarily the case that stainless steel is less hard than carbon steel – they can both be manufactured to different hardness levels. Studies performed on steel knife blades by the Cutlery and Allied Trades Research Association (CATRA) in Sheffield, England found that at hardness 61 stainless steel had slightly superior cutting performance over carbon steel.ref Ryuga’s stainless steel is HRC 55+-2 which qualifies as medium carbon steel.ref

Looking at what is available out there on the internet, it’s hard to see any data on hardness or carbon content in any of the bonsai tool descriptions. Caveat emptor applies then, since it could be any old steel composition that you are buying. It could easily be low carbon steel, which will lose the edge quickly and not be very strong, and rust easily!

Given this I would personally prefer to buy a tool which either has a specified hardness rating, or is labelled with the type of steel it’s made from so you can work out its hardness. Anything labelled high carbon, white or blue steel should work well. If you’re buying a high carbon steel tool, you do need to prevent rust, so make sure it’s cleaned and oiled after use, and stored out of the damp.

If you prefer to have some rust resistance, stainless steel is a perfectly good option and again look for something with a specified hardness in the high carbon range. Tian Bonsai has a 62 level hardness stainless steel range. Kiku appears to have 62-63 hardness stainless steel in their Gold range. Kaneshin have a range of blue steel tools (mostly scissors), and UK-based Niwaki have secateurs made by Tobisho from blue steel #2 and some blue steel scissors. I’d also consider the slightly more affordable ARS – a Japanese brand with a European distributor who produce high carbon steel secateurs and scissors and are available online.

Be prepared to fork out some significant cash for your high carbon and/or stainless steel tools, they are what one might call ‘investment pieces’. But since steel production contributes 8% of global carbon dioxide emissions, buying once and using forever is probably the best strategy.

Update: if you’re wondering what you can use to clean your tools, you can check the compatibility of carbon steel with various substances here, and stainless steel here. With carbon steel avoid anything acidic, according to these charts it can be damaged by buttermilk, gelatin, malic acid (apples), citric acid (lemons), lactic acid (milk), mustard, salt water, tomato juice, water, petroleum jelly, wine and whiskey! Stainless steel is much less sensitive.

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