Don’t know about ‘Sumitsubo’, Japan’s ink line? Let me introduce its convenient uses

Hand Tools

This time, I’ll introduce how to use a tool called ‘Sumitsubo.’ Some might not be familiar with the term ‘Sumitsubo,’ but think of it as akin to a chalk line. Instead of chalk, ink is applied to a string, which is then stretched and snapped against wood, transferring the ink from the string to the wood to draw a straight line—consider it Japan’s ink line. The Sumitsubo has been an indispensable tool in Japanese construction since ancient times, capable of drawing lines that rulers cannot, producing lines far finer and more visible than those made with a chalk line.

It’s not a difficult tool to master, but it does require a bit of know-how to use effectively. It’s incredibly handy for woodworking and crafting, so if you’re curious, I encourage you to give it a try.

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日本語版の記事（墨壺（すみつぼ）の使い方【プロの大工用】コツを知るだけで簡単修得！）はコチラ

Article Author

Kentarou Fukata
A Kyoto, Japan native born in 1985, I am a seasoned carpenter boasting five national qualifications in housing-related disciplines in Japan. Committed to the dissemination of carpentry expertise, I actively contribute to professional development within the field through instructional content on YouTube.

Introduction Video

Complementing the guide is an informative video designed to augment comprehension.

14.墨壺（すみつぼ）の使い方【技能士試験用】すぐに差が出る！

If you wish to utilize features like subtitles, please reopen the video on YouTube. You can access the subtitles through the ‘CC’ button on the playback screen or in the settings.

About ‘Sumi’ (Marking for Processing Lines)

The ‘Sumitsubo,’ also known as a ‘Sumi pot,’ is a tool used for marking ‘Sumi’ (processing lines). Understanding the role and characteristics of Sumi can be a shortcut to mastering the use of the Sumitsubo.

Main Roles of Sumi

The primary role of Sumi is to mark processing lines. Besides processing lines, it also marks baseline, directive lines, and ‘Bandzuke’ (notations of component names).

In processing work, it’s common for the person marking with Sumi to be different from the one who does the processing. It’s crucial that the processing lines are clearly understandable at a glance by the processor. Even when processing by oneself, clear and understandable marking lines improve checking efficiency and prevent mistakes.

Cleaner Sumi is Better

Buildings have long lifespans, and during renovations, the Sumi markings made by carpenters of the past often resurface. These remaining marks reveal the skill of the carpenter. Precise and clean Sumi is beautiful and pleasing to anyone who sees it. Of course, precise craftsmanship cannot be achieved without beautiful Sumi markings. Aim for clean marking, including the handling of the Sumi ink itself.

How to Use a Sumitsubo and Tips for Striking Straight

Basic Use of a Sumitsubo

The Principle of a Sumitsubo

To explain the principle of a Sumitsubo in the simplest terms, it involves pulling a string soaked in ink, pinching and flicking the inked string, so that the ink from the string is transferred onto the material.

The starting point of the string is attached to a ‘Kalko’ (a stick with a needle), which can be fixed at the starting point of the ink line. The endpoint is held down with the fingers of the hand not flicking the string. The Sumitsubo itself is equipped with a reel for winding the string, and in recent years, automatic winding types with springs have become mainstream. At the front of the reel, there’s a pot for storing ink, filled with cotton or a sponge.

Basic Striking Technique

First, fix the Kalko at the starting point of the line. At this time, wrap the string around the needle tip 2-3 times and insert it, adjusting so it doesn’t shift from the starting point. In Sumi striking, because the string is pulled strongly, insert the needle tip towards the Sumitsubo side firmly to prevent it from coming out or shifting.

On the Sumitsubo side, hold the body with the hand opposite the striking hand, pinch the string tightly so that it doesn’t get pulled out from the reel even when tension is applied, and fix the endpoint with the tip of the thumb.
※You can also fix the reel from rotating with the palm holding the body, but prioritize fixing with the fingertips.

Ink on the string dries if left untouched, so strike before it dries. Gently pinch the string with the striking hand, lift gently, and release gently.

Drawing Straight Lines on Uneven Surfaces

The Sumitsubo allows for drawing accurate straight lines even on uneven surfaces, such as log beams.

Due to the principle of the Sumitsubo, ink is applied at the intersection points between the plane passing through three points—the starting point, the ending point, and the lifted apex—and the material’s finishing surface. This principle enables the quick drawing of core lines on objects like log beams.

However, it is important to note that if the direction of the lifted apex is incorrect, an accurate line cannot be drawn. When drawing core lines, it’s crucial to adjust the position of the lifted apex so that the plane formed by the three points of string tension aligns with the plane of the core line.

Tips for Striking Sumi Straight

Use the Sumitsubo with Bare Hands

Using the Sumitsubo with bare hands is essential, as wearing gloves can lead to unintended curving. Gloves can cause the ink string to rotate when pulling or releasing it, creating a slicing effect that makes the sumi curve like a curveball.

Especially in the case of long-span sumi strikes, even a slight rotation can cause a significant curve in the sumi line. Therefore, care should be taken to release the pulled ink string without adding any rotation or twist.

Be Mindful of Wind During Outdoor Work

When using the Sumitsubo for outdoor tasks, natural wind can cause the lines to curve.

Improving Accuracy with Sectional Striking

For long-span sumi striking, where the risk of the sumi curving is higher, secure accuracy by holding down the stretched string in the middle of the span and striking in sections, front and back.

About Throwing (Bending) Sumi

While the Sumitsubo is primarily a tool for drawing straight lines, it can also create curved lines. Opportunities to need a curved sumi are rare, but understanding the principles of curving will help prevent unintentional bending when aiming for straight lines.

To bend a line, during the sumi striking process, rotate the lifted ink string with your finger (so that the upper side faces the direction of motion). When releasing, do so with the motion of ‘throwing’ the ink string in the desired direction of the curve. The released string will rotate, cutting through the air like a curveball in baseball, creating a beautiful curved sumi line.

The necessity for this technique arises from the need to adjust processing baseline considering factors like warping after processing, the weight of a roof, and fittings. This technique is challenging; it requires precision to achieve the targeted dimension in a single attempt and is not easily mastered.

Introducing Tricks Used in Skill Competitions

In Japan, there are competitions to showcase carpentry skills and skill certification exams to prove a certain level of expertise. In these events, the demand for sumi marking exceeds that of regular work, requiring it to be not only precise but also fast. Various tricks are utilized to meet these standards. These techniques, while honed in competition, are also applicable to DIY projects, so let’s take a look at some of them.

How to Make Clean Sumi Marks

When striking sumi with a Sumitsubo, excess ink can sometimes splatter around. Wiping off the excess ink from the string with a lightly squeezed rag before striking can produce sharp, pencil-like sumi lines. Regularly cleaning off the ink stains on your hands or a ‘framing square’ also prevents unnecessary mess.

Adjusting the Ink Density

In addition to the Sumitsubo, a pen called ‘Sumisashi’ is used for sumi marking tasks. The Sumisashi utilizes the ink from the Sumitsubo.

In fact, the ink concentration typically used is often too thick, and simply diluting it with water to the appropriate density can make it much more user-friendly. A good concentration guideline is to achieve a gray line, similar to the darkness of a pencil.

Diluting the ink not only reduces stains on hands and materials but also improves the ink retention on the Sumisashi, enhancing work efficiency.

Marking Multiple Short Components with Sumi

When marking the core of many components with sumi, there’s a way to reduce the number of times you need to insert and remove the Kalko, as well as the amount of sumi wiping required.

Make a notch (a groove that the string can catch on) at the core part of both ends of each component using a chisel. Then, with the Kalko stuck in the workbench, switch out the components along the string and strike the sumi.
After wiping off the excess sumi once, you can make good marks 3 to 5 times, allowing for quick and clean sumi marking.

Care Tips and Precautions for Your Sumitsubo

How to Add Water

To prevent the Sumitsubo from becoming unusable due to drying out, it’s crucial to regularly add water.
Adding water directly to the ink reservoir of the Sumitsubo can result in uneven ink adhesion and concentration, leading to poor quality marks. Instead, when adding water, ensure it is absorbed by the string in the reel section.

Precautions for Use

When using a Sumitsubo, care must be taken with the handling of the Kalko, which is equipped with a needle. If the Kalko becomes detached while the string is being pulled tightly, it can fly towards the direction it was pulled with force.
Make sure to insert the Kalko securely to prevent it from coming loose.  This is especially important when working in pairs.

Choosing a Sumitsubo

Since a Sumitsubo is frequently used in professional settings, let me share some tips on choosing one with practicality in mind.

Ease of Adding Water

During summer, Sumitsubos can dry out almost daily, necessitating regular watering. For models with an automatic reel, the ease of adding water can vary significantly between manufacturers. The Tajima company’s Perfect Sumitsubo from Japan is recommended for its ease in adding water to the reel section, including detachment and spring rewinding.

About the Color of Sumi

The fundamental color of sumi is black, but for cosmetic materials (raw wood to be finished with a plane), a vermilion sumi is used as it leaves fewer stains. Additionally, there are sumi varieties specially formulated to resist smudging on damp materials, with various types available for purchase.

Regarding the Thickness of the String

The specialized string for Sumitsubo is designed to hold ink well and has a slight stretch. This characteristic is crucial, so always use the designated string for your Sumitsubo.

I recommend using a thicker string for the Sumitsubo. While a thinner string can hold more wraps, it’s prone to fraying and breaking, and it dries out faster when marking, making it less practical to use.

Size of the Sumitsubo

Recently, compact models have become available for easy transport, and there are even specialized holders sold for them.
*Personally, I don’t see the need to carry a Sumitsubo around, so I stick to using a basic, user-friendly size.

Choosing a Kalko

Kalkos come with a safety feature to prevent accidental stabbing due to the beam part being hazardous. However, while these safety-equipped Kalkos are secure, they are known for poor accuracy and are difficult to handle, thus not favored by professionals.

A smaller Kalko can be challenging to insert forcefully since it’s pinched to stab, making larger-sized Kalkos, which can be gripped and inserted more easily, recommended. Additionally, if it does come loose, a larger Kalko moves slower, reducing the risk of injury. Kalkos with hard needles designed for concrete are also advisable. The concrete type, having a blunt needle tip, requires sharpening for use on wood.

Conclusion

Have you become interested in the ‘Sumitsubo’? It’s an indispensable ink pot in Japanese carpentry. It’s not difficult to handle, so if you’re involved in carpentry, I encourage you to pick one up and give it a try. The hand-wound type can even be made by hand, so why not take on the challenge!

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