What a slag brick is
A slag brick is made chiefly from the slag left over when iron is smelted. Unlike the ordinary red brick, which is clay fired in a kiln, it is not fired at all. A government-funded research project on the subject groups it with coal-ash brick as one of the two unfired bricks that spread through Japan in the Meiji, Taisho and Showa eras, and is studying how they were made at the state-run Yawata ironworks and at Kamaishi.
The colour gives it away. The Agency for Cultural Affairs describes the brick wall of the former Yoshihara residence in Okawa, Fukuoka Prefecture, built in the late Taisho era, as laid in grey-brown slag brick, and that muted grey cast is the first thing to look for.
Kitakyushu City presents the office building of the former Sapporo Beer Kyushu brewery in Moji, completed in 1913, as one of the few surviving buildings in slag brick, and the national register also singles out its use.
In the records, and what to look for
The structure field usually says only brick, leaving the slag to the descriptive text. The water tower of the Hisatsune residence in Nakatsu, Oita Prefecture, from 1924, is recorded as brick, 7.6 m high, with the description adding that its walls are a single brick thick in slag brick. Only call a building slag brick where a description like this supports it.
The material turns up in office and warehouse walls, garden walls and water towers. At the former Sasebo Naval Arsenal torpedo range in Kawatana, Nagasaki Prefecture, a 1939 extension raised the stone lower walls with an upper storey in slag brick.
On site, look at colour and texture together. Fired brick is an even red; slag brick runs from grey to grey-brown and may show grains of aggregate. Bond patterns are described in the usual terms, such as English bond.
References
- KAKEN Grants-in-Aid for Scientific Research, project 23K26249: reconstruction of coal-ash and slag brick manufacture (Japanese)
- https://kaken.nii.ac.jp/grant/KAKENHI-PROJECT-23K26249
- Agency for Cultural Affairs, Database of National Cultural Properties: Former Sapporo Beer Kyushu Brewery Office Building
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00006284
- City of Kitakyushu: Former Sapporo Beer Kyushu Brewery Office Building (Japanese)
- https://www.city.kitakyushu.lg.jp/contents/02100275.html
- Agency for Cultural Affairs, Database of National Cultural Properties: Former Yoshihara Family Residence Side Gate and Brick Wall
- https://kunishitei.bunka.go.jp/heritage/detail/101/00011586
- Agency for Cultural Affairs, Database of National Cultural Properties: Hisatsune Family Residence Water Tower
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00002337
- Agency for Cultural Affairs, Database of National Cultural Properties: Former Sasebo Naval Arsenal Kawatana Torpedo Range Machine Room and Torpedo Adjustment Room
- https://kunishitei.bunka.go.jp/bsys/maindetails/101/00015108
Last verified: 2026.10.04