Furo, urushi-buro, urushi-muro, curing cabinet – whatever you call it, you need it if are thinking about starting any urushi work*. Furo let us keep three important factors of curing urushi under control:
- Humidity – stable and controlled relative humidity – typically in range from 65 to 75%
- Temperature – stable and controlled temperature – typically in range from 20℃ to 25℃ (or 68℉ to 77℉or 293 to 298 kelvins)
- Protection from airborne dust and lint which can attach to wet urushi and mess with curing or surface quality
It can extremely simple and small but also very complex and large structure. Let’s go through several steps of furo complexity.
Basic, rudimentary furo for fuki-urushi
If you want to lacquer a small wooden object with fuki-urushi technique, and do not plan to more complex or a lot of pieces – a cardboard box furo is a cheapest and easiest option. You just need a cardboard box (with lid or flaps), a few pieces of wooden slats, a small bowl for water and maybe a wet rag. Choose the box larger than the piece you want to cure – multiply each dimension of the object by 2 and it gets you reasonable minimum (but don’t go too large – it will be more difficult do have proper humidity). Put the slats and bowl with water at the bottom, add the wet rag on one the walls, put the lacquered object inside and close the lid. That’s it. Water and wet rag in such small space will bring humidity higher, dsut and link with fuki urushi is not very important, temperature will be same as ambient – so just make it above 20℃ and you are good to cure. What are the downsides? It’s small, it’s easy to destroy, it will not take prolonged humidity well, you can’t control the humidity.
When do you need more? In two basic cases: either you plan to work on more items at the time, larger items, more frequently, or you want to try other more sensitive techniques – roiro, iro-urushi, tamenuri, etc. It leads us to two options: larger and more permanent but still basic furo, or still small but much more controllable furo.
Larger, permanent furo for fuki-urushi
Principles are the same – you need a box, temperature and humidity. So let’s just make the box bigger. For my fuki-urushi furo I use old wooden cupboard. I got it for free from neighbor. It is lacquered so the wood the box is built from does not serve as humidity stabilizer, so I added wood boards on the inside. Important note – you need wood which does not rot and is mold resistant. In Japan they use cypress, I used cedar. Make sure the cabinet is reasonably sealed – no visible openings, cracks, crevices, and that the door close well. If necessary – seal the cracks with silicone, add a rubber seal around the door opening. On the bottom add some kind of rack – I used just two long wood slats, and added easily removable “shelf”. I can cure in this furo anything from 50 sets of chopsticks, through 12 sets of wooden cutleries to many large wooden bowls or even a skateboards. If you want/need something even bigger – choose a cabinet with sliding doors – moch more convenient in furo. Not to mention – traditional 😉
For humidity – again same as before – a container with water, something to increase evaporation: a rag works well, but flat sponges are great too. If you line the interior with cedar well – even spraying the wall with water can do the trick. If you keep furo in cooler place – make sure to increase temperature – heating matts used in terrariums are pretty cool, and do not draw to much electricity. In case of such furo it is good to have some basic hygrometer – anything from Amazon around $20 or more is good enough (in case of fuki urushi). Do not expect it to be super precise. Even if it declares 3% accuracy – it’s not true in 90% of cases. But it does not matter provided it works 😉
Small furo for more demanding work
In this scenario best solution is to upgrade to plastic container with good lid. You can set it up with opening facing up or put on the side like mine. In this case lining interior with wood is good idea too. Wood lite cypress or cedar work as humidity stabilizers – they suck water out of the air and give it back, making the fluctuations of humidity smaller. If you practice and your boxes are (reasonably) airtight – you can learn how to keep parameters stable without any special tools like humidifiers. I think they are much better in big furo chambers and all machines I tested are to aggressive for furo smaller than 0.3 cubic meter. My chambers, if not opened drift 1-2% humidity overnight. Good enough for any urushi work.
As we will be after much narrower humidity ranges – much better hygrometer/ thermometer combo is a must. Decent ones start at $50. If you want real 2-3% accuracy – you need calibration which is typically much more expensive than the sensor itself and should be done periodically. But from my experience it is not necessary. I use central station with additional 8 wireless probes in each furo. I have reading of ambient in my studio and fast access to parameters of each of my furo chambers.
Those 8 probes leads us to another upgrade:
Different parameters for different curing
In my work I need at least 3 different levels of humidity. I work on many pens at once, and with different techniques, so some need 60% humidity (early stage of kawari-nuri structure curing, white urushi, etc) through typical 70-75% for many different uses, to 85% for uwazuri or fast, short setting of gold pasting urushi. Add “ambient” furo (45-50%) where I keep freshly lacquered pieces with final layer to let urushi “settle” and smooth the brush marks, but still protected from dust, and sometimes additional humidity level – and you can see why I have 6 chambers and seventh fuki-urushi furo. Keeping stable temperature is again important too, and using some kind of automation is a good idea – parts dedicated for terraria are very useful here. Alternatively – you can control temperature of entire room, like I do, with AC. Added benefit – more advanced units let you control the humidity to some extent too. Believe me – lacquering with some kind of lacquer in 80% or more ambient humidity (winter in Provence) can be tricky.
And this is something I am working on, for now designing and sourcing materials. Large, dual chamber furo, made entirely of wood, in very traditional Japanese design, with at least one rotary shelf. Rotary shelf allows for constant or periodical changes in orientation of cured pieces, preventing urushi from accumulating. It means also that holders must be compatible with it (so pens will not fall). But this I will leave for future posts.
*) Well, this not 100% true – it is possible to cure urushi in ambient temperature and humidity, including lacquered floors, walls, ceilings, provided this ambient environment is good for curing urushi type you use in terms of temperature and humidity, and ideally – low level of airborne dust and lint.