My coffee mug broke earlier this year. Naturally, you wouldn't be reading this if it weren't avery special mug: It has accompanied me every day for the last 10 years. Through my student days, startup days, and early career, it has sat by my side with its goofy crocodile decals and brightened my desk. We've shared several thousand cups of coffee. That was until early February 2024. In a moment of inattentiveness, I knocked it off the kitchen counter and, giving into the idiotic reflex of trying to catch it with my foot, kicked it into the wall instead. I can still hear the sound of it shattering...

After recovering from the initial shock and collecting the sad remains of my mug, several colleagues offered their condolences and suggested I look into the ancient Japanese art of Kintsugi. I did, and four months later, my coffee mug is back in action! I wanted to write this post to share what I learned along the way.

Kintsugi is the traditional Japanese art of repairing broken pottery. In Kintsugi, one uses natural lacquer mixed with various ingredients such as wood powder, polishing compound, and even bread dough to repair cracked pottery. The mended joints are then highlighted by dusting the final layer of lacquer with gold or silver. We Westerners might find this last step peculiar: why would you highlight a repair? In fact, why bother repairing a broken mug at all? Shouldn't a broken thing either be discarded and replaced, or repaired to look as good as new? According to traditional Japanese philosophy, however, an object's imperfections are to be celebrated. They tell the story of the object’s history.

With the shards of my beloved mug safely stowed away, I started reading about Kintsugi and how one might go about practicing it at home. These days, some hobbyists opt for modern epoxy instead of the traditional and expensive Urushi lacquer. Epoxy probably yields a more robust bond and certainly allows for quicker repairs, but I question its safety for food-related use, especially at the temperatures found in a steaming cup of coffee. Also, the idea of using epoxy didn't feel right to me—I wanted to do this the proper way or not at all. That meant investing many yen to obtain what is widely regarded as the best Kintsugi starter kit made by the folks at TSUGU TSUGU. While waiting for overseas shipping, I familiarized myself with the process by watching their tutorial over and over again.

When your pottery has shattered into different pieces, the first step is naturally to glue them together again. Before gluing, the pieces need to be thoroughly cleaned and any sharp edges smoothed so that the seams become more pronounced. This aids the later steps in the process.

Already here I encountered my first hurdle. The guides I was following worked with clay pottery, while my coffee mug is glazed ceramic. In case you didn't know, ceramics are surprisingly resilient; sanding it isn't easy. I had coarse 200-grit sandpaper at hand, but a diamond file would most likely have been a much better tool for this job.

Mistake 1: Sandpaper doesn't work on glazed ceramics. Use a proper diamond file.

After smoothing the edges, it's wise to test-fit the pieces together: I rebuilt the cup from its fragments using some masking tape. This allowed me to get a feel for how they fit together, which pieces I was missing (luckily not many), and which pieces were too small to handle. You can rebuild a surprising amount of the object's structure with kintsugi techniques, so leaving small shards out of the repair is not a big problem.

All the pastes, glues, and lacquer products used by kintsugi artisans are based on the resin obtained from the Urushi tree. Urushi lacquer contains urushiol, an organic compound named after the tree from which it is harvested. Incidentally, urushiol is also what makes poison ivy... poisonous. Most humans are allergic to this compound. Gloves are definitely recommended when working with urushi resin.

Urushiol hardens to form a very hard and scratch-resistant lacquer under the right conditions: around 25°C and 75% relative humidity. These conditions can be simulated by placing a wet towel in a cardboard box and leaving it in a warm place. I managed to convince my fiancé that this cardboard box just had to live in the bathroom cupboard and that the hairdryer now lives elsewhere and yes this is super critical because it's my passion project right now. I kept tabs on the environment of my urushi box using the world's best CO₂ sensor (full disclaimer: I work at Disruptive Technologies) to make sure I was alerted when the conditions deviated too much from my target.

The glue used to stick broken pieces together in kintsugi is called mugi urushi. Strangely, it's made by first mixing cake flour with water and forming a small ball of dough. I suspect the gluten in the dough helps achieve the right consistency. The dough is then mixed with small amounts of raw urushi until it reaches the consistency of freshly chewed gum. At least, that's what the kintsugi guide told me. I was surprised when I suddenly had something indistinguishable from brown slimy chewing gum and hoped I was on the right track.

Using a suited poking implement (the kit came with a well-shaped bamboo stick), I smeared the sticky glue onto the edges of some of the larger shards and pressed them together. Mugi urushi is sticky enough that pieces will hold together fairly well, making it easy to fasten them properly using small pieces of masking tape. The guide recommends not gluing more than a few pieces together at once and to focus on larger pieces to begin with. Following this advice, I planned to glue my mug together in two sessions.

Once glued and taped, the broken pieces were placed into the humid and warm cardboard box for around 2 weeks. At this point, the glue had hardened to the point where scraping at the squeezeout with a razor blade caused it to chip and flake off. If the glue feels gummy, leave it in the urushi box for another week -- it will harden eventually. If it doesn't, the mugi urushi might have been contaminated with oil and you will have to start over. Dissolve the mugi urushi with acetone and restart. I luckily did not encounter this particular problem, but that's what Reddit recommends.

The 2nd round did not go as well. I quickly realized that my first glue-up was slightly crooked.

Mistake 2: Even the smallest misalignment will haunt subsequent glue-ups. Take care and plan well.

I did not test-fit the first glue-up with the other pieces before letting it harden because I was afraid to smear the squeezeout and smudge the ceramic surface. I now realize that this wouldn't have been a problem, because you can easily sand away any smearing on glazed items. For raw unglazed (rough) pottery, this is probably not the case though. In any case, the 2nd glue-up did not go very well because of the misalignment with the first. In hindsight, I should have started over at this point, but oh well.

After gluing the broken pieces together, it's time to start rebuilding any missing structure. It's hopefully evident from the photos that I'm missing a fairly large chunk near the bottom of the mug as well as a couple of fragments near the lip. There's even a hole straight through the ceramic!

You can also see the misalignment resulting from my haphazard glue-up in that the rim of the cup isn't flat. A true kintsugi artist would never let such a blemish fly. Luckily, I can go full meta and say that the misalignment tells a story of my kintsugi journey, and the mug will be that much more valuable for it. After all, that's the spirit of this whole project!

Luckily, there's a process for dealing with (and celebrating) defects like holes and missing pieces. Large holes deeper than 1 mm are filled with kokuso. Kokuso is made by taking mugi urushi (the gum glue used earlier) and mixing it with equal quantities of wood dust and polishing compound. At this point I simply trust the process—I couldn't begin to tell you why these particular compounds are used.

Once mixed, kokuso has the consistency and appearance of hard clay. I worked kokuso into the cracks and holes in 0.5 mm layers. Kokuso can be flattened by covering it with plastic clingfilm and pressing on it with your finger. Between each layer, kokuso is allowed to harden for one week. Once cured, it is sanded flush with the contours of the surrounding geometry.

I needed two layers of kokuso to rebuild most of the structure, and the 2nd layer took several weeks to fully cure. I believe this is because I did not clean my tools well enough. The kintsugi guide recommends cleaning all tools with mineral oil between sessions precisely because oils will prevent urushi from curing, and I suspect I did not do a proper job of cleaning the oil off my spatula when making the 2nd round of kokuso.

Mistake 3: Even the smallest amount of oil will significantly prolong cure time.

Small holes and cracks shallower than 1 mm are filled in using sabi urushi. This is made by mixing polishing compound with water, little by little, until it reaches the consistency of hard mayonnaise. To this, you add raw urushi (around 60% by volume). The guide says that sabi urushi hardens "quickly" and boy is that an understatement. In less than 10 minutes, my sabi urushi was hard and useless. I had to practice making sabi urushi a couple of times until I had the process nailed before I even had time to apply it to my cracks. Unsurprisingly, I found the ticking sabi urushi clock to be very stressful, so I will admit that I did not spend enough time on this part of the process.

Mistake 4: Not spending enough time on the sabi urushi process.

Had I done my job properly, I would have sanded down the hardened sabi urushi and added several layers before moving on to the next part of the process. I did not, and proceeded with several small holes and gaps in my seams. These were not deep enough that I feared my mug would leak, of course, but it's still a visual blemish.

The next step of the process is where the detail work begins. At this point, the gaps are all filled, the cracks are smoothed over, and we are ready to start working on the final surface finish. That's done by repeatedly painting layers of black urushi lacquer on top of every part of our repair.

Black urushi is made by stirring a blob of raw urushi lacquer until it has evaporated much of its water content. During this process, it transitions from a dull beige color to deep wine red. To this, we can add black mica powder to color it jet black. I don't think its color matters much at this stage in the process. Using a fine tip paintbrush, I carefully painted the lacquer on top of the sabi urushi. To my surprise, this was the most difficult part of the whole process, especially painting the inside of the mug. Save for painting houses, I have never used a paintbrush in this capacity, nor do I have any drawing skills.

Mistake 5: Never learning to hold and use a fine-tipped paintbrush.

Sure, I can trace a line with a ballpoint pen, but pens work in two dimensions. Paintbrushes work in at least three: pressure widens the brush and curves cause it to splay. Stray hairs muddle your line and you must always be mindful of the amount of lacquer (or paint) your brush is loaded with. I found it near impossible to paint nothing but the thin lines of sabi urushi. This is without a doubt the part of the process that I underestimated the most. I have gained an enormous amount of respect for artists who paint for a living.

I laid down several layers of black urushi, making sure to cure each layer for a couple of days and wet-sanding it smooth, to give myself some practice with the frightfully difficult paintbrush. I noticed that the black urushi tended to break off in flakes where I had painted directly on the ceramic glaze rather than on the thin strip of sabi urushi that makes up my seams. Apparently, urushi lacquer doesn't bond very well to the glaze, and I believe I should have either (a) broken the edges of the ceramic more thoroughly prior to gluing to widen the seam or (b) pre-sanded the glaze along the seams.

Mistake 6: Not providing a scored base for the black urushi to bond with the ceramic.

This is the finish line! Several months of mixing exotic compounds together, painting, brushing, sanding, and a lot of waiting for urushi to cure have led up to this moment. The final layer of lacquer is applied at this stage. I'm going for a gold finish on my mug, and the recommended base layer for this is bengal red urushi. Painting or sprinkling gold on top of red or earthy materials is apparently an old trick to improving the overall color of the final result.

Bengal red urushi is made in exactly the same way as black urushi lacquer, but uses red mica powder instead of black. The process of painting it is also the same—i.e. frightfully difficult, even more so because this is the final layer and there is nowhere to hide your mistakes. No pressure, hope you have steady hands.

After painting the bengal red base layer, you want to let it sit for a while: it must be tacky but not liquid. 15-30 minutes, ish. Yeah, this was also a point of worry for me. Experienced kintsugi artisans can probably glance at the bengal red and know that it needs a few more minutes before it is ready for gilding. I don't, I only have data: I spent 15 minutes laying down the bengal red base coat, I waited exactly 30 minutes after the last brush stroke before applying the gold powder. The room I worked in was at 25.3°C with a relative humidity of 71.3% and the CO₂ levels hovered around 480 ppm. There was a full moon and a thunderstorm, in case you wondered. After 30 minutes, the bengal red urushi was just a tad too liquid when I started applying the gold powder.

Mistake 7: Not waiting long enough before applying gold powder and smearing the lacquer.

A significant driver of the cost of the kintsugi kit I purchased was the small amount of astonishingly fine gold powder that came with it. Included were also a couple of balls of pure silk (because why not) that, when coerced into small balls, will hold a small amount of gold powder. Carefully, I brushed the gold-laden silk balls across the lines of bengal red urushi and watched as they transformed, in the blink of an eye, from earthy red to brilliant gold, as if by magic. That is, until I applied a bit too much pressure and smeared some of the lacquer and made a mess of a couple of parts of the seams. All in all, though, I believe this part went okay. Far better than I feared, at least.

After a couple days in the urushi box, this is what I was greeted with. It's not perfect by any stretch of the imagination, but I'm glad I had the opportunity to dip my toes in the art of kintsugi. I'm also very excited to finally have my mug back.