Lime plaster is made by mixing burnt lime with water and letting it harden by drawing carbon dioxide from the air. A plastered wall, in other words, slowly turns back into the substance of stone. From the plasterer’s trowel to the gypsum reliefs of Western-style ceilings and the modern work that keeps those ceilings safe in an earthquake, we read what lies behind the white surface.
A white wall made of the stuff of stone
The white walls of old storehouses and castles, the ceiling of a Western-style hotel lobby: many are finished in shikkui, Japanese lime plaster. Shikkui is a plastering material made mainly of slaked lime, mixed with water together with fibres such as hemp (called susa) and seaweed glues such as funori or tsunomata. Despite the character for lacquer in its name, it has nothing to do with lacquer; the name is said to come from an old reading of the word for lime.
Each ingredient has a clear job. The fibre prevents cracking and adds strength; the glue gives body and holds water, making the plaster easier to spread. Sand or clay may also be added.
What makes lime plaster interesting is how it sets. It does not harden by reacting with water, as cement does. Once applied, it takes in carbon dioxide from the air, and the slaked lime (calcium hydroxide) turns into calcium carbonate.
Slaked lime + carbon dioxide → calcium carbonate + water
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
Calcium carbonate is also the main component of limestone. Limestone is burnt to make quicklime, water is added to make slaked lime, and the lime is mixed and spread. The finished wall draws in air and returns to the same substance as limestone. A plastered wall sits partway around a cycle that begins in stone and ends in stone.
What it means to set slowly
Materials that harden by reacting with air are called air-setting. The carbonation of lime plaster is slow, and it takes many years after application to harden fully. Researchers even use “accelerated curing” in carbon-dioxide-rich chambers to speed it up.
Slow drying translates directly into labour. The plaster must be built up coat by coat, each allowed to dry a little, and the work takes time. Its softness and weakness against water have long been counted as drawbacks; wet plaster is heavy and more likely to come away.
On the other hand, lime plaster has qualities well suited to living with. It takes in and releases moisture, insulates, and resists fire. A traditional dozo storehouse was a fireproof building whose walls were built up in clay to a thickness of 20 to 30 cm, then finished in plaster. Research has also shown that it softens odours and adsorbs and breaks down formaldehyde.
With a trowel, one coat at a time
Plaster is applied by a craftsman with a trowel. A scratch coat, an evening coat, a brown coat and a finish coat are laid one after another, each left to settle before the next. The lower coats are pressed firmly into the base; the upper coats are thinner and smoother.
Western-style ceilings often used a base of wooden lath: thin strips nailed up with small gaps between them, into which the plaster is pressed. The plaster that squeezes through the gaps hardens behind the lath and hooks the ceiling in place. According to research at Kogakuin University, specifications of the period gave no figure for the amount of mixing water; the stiffness of the mix was left to the craftsman’s feel.
The trowel does more than flatten surfaces. In the mid-Edo period, kote-e emerged: relief pictures and crests modelled in coloured plaster. Chohachi Irie (1815–1889), born in Matsuzaki on the Izu Peninsula, studied Kano-school painting in Edo and perfected a richly coloured kote-e technique. His best-known work is the “dragon that glares in all directions” in the main hall of Jokanji temple.
Gypsum ornament on Western-style ceilings
From the Meiji era onward, many Western-style buildings in Japan combined plaster surfaces with cast gypsum reliefs. Gypsum plaster is made mainly of calcined gypsum and sets differently from lime: it is hard, strong and quick-setting, and cracks less as it dries. Being able to cast many identical ornaments in a mould is another of its strengths.
The main building of Hotel New Grand in Yokohama (1927, Jin Watanabe) is a prime example. The ceilings of its second-floor lobby and banquet halls are decorative suspended ceilings with a lath-and-mortar base, a plaster finish and gypsum reliefs. The arched ceiling of the “Rainbow Ballroom” is said to be the masterpiece of the plasterers of its day.
The white trowelled surface and the shadows of cast gypsum: two materials making up for each other in a single ceiling.
Keeping heavy ceilings safe in an earthquake
Beautiful ceilings have a weakness: they are heavy. In the Great East Japan Earthquake of 2011 there were some 2,000 cases of ceiling damage. In response, the Building Standards Act’s enforcement order was revised in April 2014, and suspended ceilings meeting certain conditions became “specified ceilings” subject to new safety standards.
The new standards are not applied retroactively to existing ceilings, which are treated as “existing non-conforming” (City of Yokohama).
The renovation of Hotel New Grand is known as a careful answer to this problem. The ceilings had originally been hung with bamboo and copper wire. In the renovation, the existing gypsum reliefs were taken down, the ceiling surface and its base were strengthened, and the reliefs were returned to their original places. The removed reliefs were joined with stainless steel rods and backed with glass fibre before being refixed, and the hangers were replaced in stainless steel.
The preliminary survey was thorough: core samples, X-rays and 3D scans, and for the cramped ceiling void, a model tank carrying a scanner and camera. Strengthening the ceilings while keeping the main components and the original methods required a newly developed approach, and full-scale shaking tests confirmed that the ceilings would not be damaged in a major earthquake.
- 1927The main building opens, with plaster and gypsum-relief ceilings
- 2011Great East Japan Earthquake; ceilings fall across Japan
- 2014Enforcement order revised; standards for “specified ceilings”
- 2016Renovation including seismic work on the ceilings completedOctober
Look up when you stay
A plaster ceiling changes with the light. In the morning, when sunlight enters low through the windows, the reliefs cast their deepest shadows; under the evening lights, the white surface returns a soft glow. Look up at the same ceiling twice, in the quiet hours when the lobby is empty.
When you do, notice the line where flat surface meets ornament. A trowelled plane and a cast relief return light a little differently. Once you see it, you see two kinds of handwork layered in one ceiling.
Where to see this technique
- Hotel New Grand (Yokohama, 1927, Jin Watanabe): plaster and gypsum-relief ceilings in the 2F lobby and banquet halls
Sources
- Digital Daijisen, Heibonsha World Encyclopedia, Britannica Japan: “Shikkui” (Kotobank)
- Sawabe and Torigoe, “The culture and chemistry of shikkui”, Kagaku to Kyoiku 64(3), Chemical Society of Japan, 2016
- Tanaka et al., “Carbonation and properties of plastering materials, part 1”, Nihon University
- Kuwano Laboratory, Institute of Industrial Science, University of Tokyo
- Japan Society of Finishing Technology, 2021 conference paper (Kogakuin University, Goto et al.)
- Ministry of Land, Infrastructure, Transport and Tourism, ceiling-collapse measures; City of Yokohama, “Specified ceilings”
- Japan Federation of Construction Contractors, seismic retrofit case (Hotel New Grand); BELCA Award materials; Kanagawa Shimbun, 4 October 2016
