Handmade matters here for a practical reason. A factory tooled for volume makes one blade and sells it to you as soft or hard. One person grinding blades by hand can make three thicknesses and tell you what they are.
Wolsoo Song makes potter's knives at 0.2mm, 0.4mm and 0.6mm, in a basement workshop in Eunpyeong-gu, Seoul. He grinds every blade himself, and he is a ceramist, so each one is worked out around its shape, thickness, flexibility and feel in the hand. Four tenths of a millimetre separates the thinnest from the thickest, and it makes the thickest roughly twenty-seven times stiffer.
The three thicknesses in the same bowl wall. Photograph: SPD Tools.
A hand-ground bevel can also be wrong. A factory sets the jig once and runs it for a year. Song looks at every blade, and the edge you get is the one he settled on that day.
Twice as thick, eight times as stiff
How much a blade bends under your hand is governed by its second moment of area, and for a flat section that quantity scales with the cube of the thickness. Width and steel being equal, double the thickness and the blade becomes about eight times stiffer. Triple it and it is about twenty-seven times stiffer.
- 0.2mm to 0.4mm is roughly an eightfold jump in stiffness.
- 0.4mm to 0.6mm is roughly three and a half times again.
- 0.2mm to 0.6mm is around twenty-seven times.
In the hand they feel like three different tools.
What flex actually does in clay
A flexible blade loads up against the work. Push it into a curve and it bends to the curve, which suits following a form, shaving a surface or cutting a line round the outside of a pot. A stiff blade goes where you put it and leaves a flat wall behind it: a straight cut through a slab, openwork, anything where a wandering line would show.
The trade has always sold this as soft versus hard fettling knives. Here you get the number instead of the adjective, and you can own more than one point on the scale.
Starting points:
- 0.2mm. Most flexible. Fine detail, thin walls, shaving and refining, work where you want the blade to give.
- 0.4mm. The middle, and the one to buy first if you are buying one.
- 0.6mm. Stiffest. Slabs, straight cuts, openwork, anywhere you need the line to stay where you put it.
Photograph: SPD Tools.
Straight or curve
Two blade shapes, and the maker is specific about them.
Straight is for straight cutting. It gives directional control for clean straight cuts and for openwork.
Curve is for curved cuts and surface refinement. It follows natural curves smoothly, which suits shaping and refining a surface.
Photograph: SPD Tools.
Combined with thickness, that is six tools. Nobody needs six. Most people need one, and the sensible first buy is a 0.4mm in whichever shape matches the work you actually do.
Potter's knife, fettling knife, needle tool, wire
The words get used loosely, so for the record.
A potter's knife is a blade for cutting and trimming clay, typically with a long tapered point for getting into a form.
A fettling knife is the same family. The name comes from fettling, the job of cleaning up a cast piece: trimming the seam left by a mould, tidying greenware before it is fired. Any potter's knife will fettle, and the terms are now used interchangeably by most suppliers.
A needle tool pierces and scores. It makes a point, not a line, and it is the wrong tool for a cut of any length.
A wire cuts through a block or off the wheel. A knife through a thick block gives a jagged wall. Use the wire for bulk and the knife for the cut that shows.
The maker
SPD Tools is one person. Wolsoo Song designs, grinds, sharpens and finishes every knife himself, and he works in clay, so the range is built around shape, thickness and flex.
Grinding a blade, freehand. Photograph: SPD Tools.
That is a bare hand holding a blade against a moving abrasive belt. No jig. The bevel on the knife you buy was ground by eye. When we asked about UK exclusivity he told us his production volume is limited because he makes each tool by hand, and that he cannot supply many distributors at once.
Marking a handle. Photograph: SPD Tools.
Korean tools
When Toyotomi Hideyoshi invaded Korea in the 1590s, Korean potters were taken to Japan. They were put to work in Hizen, the province covering what is now Saga and Nagasaki, and what they established there first was Karatsu ware: stoneware, often slip-decorated or iron-painted, built on techniques brought over from the peninsula. Karatsu became the everyday pottery of western Japan and it is still made.
Porcelain came after. Fukaumi Soden and his wife Hyakubasen were among those potters. They settled at Takeo and made pottery at the Kotoge kiln, and when porcelain stone was found at Arita, Hyakubasen moved the family there. They are counted among the founding families of Arita ware. We wrote about their descendants, who still make the cobalt blue Arita is painted with, in Gosu, the Japanese underglaze behind Arita blue and white.
So the order runs Korean potters, then Karatsu, then Japanese porcelain. None of which tells you anything about this particular knife, beyond the fact that the peninsula has four hundred years of form in making things from clay and the tools to do it with.
Which one to buy
If you are buying your first: 0.4mm. Straight if you mostly cut slabs and do openwork, Curve if you mostly throw and refine surfaces.
If you already own a stiff knife and want something for detail: 0.2mm.
If you already own something flexible and it keeps wandering on you: 0.6mm.
Looking after it
Stainless steel, so it will not rust the way carbon steel does. Dry it anyway.
Do not use it as a scraper or a lever. A thin blade is thin in every direction, and the 0.2mm in particular will take a set if you twist it.
It arrives sharp and it arrives in a clear sheath. Keep the sheath on it in a tool roll, for your fingers more than for the blade.
Questions people ask
What is a fettling knife used for?
Cleaning up cast or hand-built greenware: trimming mould seams, levelling uneven surfaces, tidying joins between coils or slabs. It also cuts slabs and leather-hard clay.
What is the difference between a fettling knife and a potter's knife?
In practice, nothing. The names come from different jobs, fettling a cast piece versus general cutting, and suppliers use them interchangeably now.
Should a clay knife be flexible or stiff?
Flexible if you want it to follow a curve or shave a surface. Stiff if you want a straight cut that stays where you put it. If you can only have one, something in the middle.
What thickness should a potter's knife be?
These are sold at 0.2mm, 0.4mm and 0.6mm. 0.4mm suits most people for most work. Thickness matters more than you would expect, because stiffness scales with the cube of it.
Can you cut a block of clay with a knife?
You can, badly. The blade drags and the wall comes out jagged. Use a wire for bulk and a knife for the cut that shows.
Will it rust?
It is stainless. Dry it after use anyway.
Can you sharpen it?
Yes, but gently and rarely. These arrive sharp and the bevel was ground by hand; a coarse stone will do more harm than the dullness it is fixing.
Is it worth £38 to £48 for a knife?
Depends how much cutting you do. One person grinds each blade by eye in a Seoul basement, and the price reflects what that costs him in time. If you cut clay occasionally, a cheap fettling knife is fine.
What we stock
Six variants of the SPD Tools handmade potter's knife, Straight or Curve in 0.2mm, 0.4mm and 0.6mm, told apart by handle colour: white for 0.2mm, yellow for 0.4mm, black for 0.6mm. Each comes in its own box with a clear sheath. £38 to £48 depending on thickness.
Where to go next
- Why trimming tools stop cutting, on the other edge material in the shop and why it chips instead of going blunt.
- Gosu, the Japanese underglaze behind Arita blue and white.
Sources and credits
Photographs of the knives, the cuts and the workshop are by SPD Tools and used with their permission. The guidance on each blade shape is the maker's own. The blade-stiffness arithmetic is standard beam theory: the second moment of area of a rectangular section scales with the cube of its thickness, so bending stiffness does too.
