Repairing my mug with Kintsugi

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.

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.