Oya Museum

The museum is an underground museum that was built on the site of an Oya Stone quarry. Even if you are not familiar with the name Oyaishi, the texture of the stone should be familiar to you.

This stone has long been used as a building material for houses and walls, but where and how it is mined is a valuable experience that can only be seen here.

In the museum, there is an exhibit that shows the history of Oyaishi mining. Materials from the mid-Edo period to around 1959 are on display, including tools from the hand-drilling era, mining methods, and changes in transportation.

The highlight of the museum is the underground mining site. It is a huge space covering an area of 20,000 square meters, and is 30 meters deep, reaching 60 meters underground at its deepest point.

The average temperature inside the mine is usually around 8 degrees Celsius. There are approximately 10 million quarried stones, and it is truly like a huge underground structure.

In this fantastic underground space, concerts, plays, shows, Noh plays, weddings, and other events are held. It is also used for filming movies and dramas.

The ruins of the Oya Stone quarry and the surrounding greenery blend harmoniously to create a unique landscape.

The main exhibits are as follows

Geology of Oya
Mining method and form of Oyaishi
Ruins of Oya Stone Underground Mining Site (huge underground space)
Transition of stone removal and transportation

Furthermore, in addition to the general exhibits, cultural events such as concerts, sarugaku performances, movie nights, theater performances, art exhibitions, dance performances, and seminars are held.

In addition, movies, TV dramas, TV commercials, and promotional videos are filmed here.

Every year, the site is also used as the venue for festivals at Oya, and events such as projection mapping are held in the underground space.

Regarding the history of Oyaishi and the underground mining site, Oyaishi was used as material for sarcophagi during the Kofun period (burial mounds), and in the 8th century, Oyaishi was used in the construction of Shimono-kokubunji and Shimono-kokubuninji temples.

In addition, the Otani Temple in Utsunomiya City has a Buddhist image of a Buddha made in the Heian period (794-1185), and its principal image, the Thousand-Armed Avalokitesvara, was carved from Otani stone.

Oyaishi is a pumice tuff stone mined in the Oya-cho area northwest of Utsunomiya City. Because of its flexibility and ease of processing, it has long been widely used as a building material for exterior walls and storehouses. Even today, it is used for the walls of warehouses.

The matrix of Oyaishi is mainly composed of float glass, plagioclase, and quartz, with a small amount of biotite hornblende pyroxene. It also contains silicic acid, ferric oxide, aluminum oxide, manganese oxide, lime, magnesium oxide, potassium, and sodium.

Oyaishi is thought to have been formed in the first half of the Cenozoic Tertiary Miocene Era, when the Japanese Islands were still underwater. Volcanic ash and sand and gravel erupted from volcanoes and precipitated into seawater, which solidified to form Oyaishi.

In the vicinity of Oya Town, the distribution of Oyaishi extends 4 kilometers from east to west and 6 kilometers from north to south. There are 12 quarries as of 2009, with an annual shipment of about 20,000 tons, and estimated reserves of about 600 million tons.

While open-pit mining is used in some areas, most of the quarries are dug underground at depths ranging from several tens of meters to more than 100 meters. At its peak in the 1960s, there were about 120 quarries active.

Many stones similar to Oyaishi are distributed in Tochigi Prefecture and are called Nagaokaishi, Fukaiwaishi, Iwafuneishi, Motegiishi, etc., after their respective places of origin.

Oyaishi is light and soft, making it easy to process, and has excellent fire resistance and moisture proofing properties. As a result, Oyaishi is used in a wide variety of applications, such as for fire escapes, stone walls, firewalls, gateposts, and paving stones.

It is also used as a structural material for bread and pizza ovens and stone cauldrons due to its high fire resistance and heat storage capacity. In the field of rock engineering, it is also used for experimental samples as an easy-to-handle material.

In the production area around Utsunomiya, it has been confirmed that Oyaishi was used as furnace stone in pit buildings during the Jomon period. Oyaishi is widely used for the exterior walls of stone warehouses and other buildings, railroad station platforms, stone walls, stairs, and gateposts.

The gyoza (dumpling) statue installed at Utsunomiya Station in conjunction with a TV program and the Utsunomiya Catholic Church (commonly known as the Catholic Matsugamine Church), built in 1932, are also constructed of Oyaishi. Oyaishi has also long been used in the construction of local historical buildings such as Shimono-Kokubunji Temple and Utsunomiya Castle. The unique texture of Oyaishi became widely known when American architect Frank Lloyd Wright used it in the former main building of the Imperial Hotel in Tokyo.

Immediately after it is quarried from the ground, Oyaishi has a bluish color due to its high water content, but as it dries, it turns brownish white.

The brown spots on the surface are called “miso,” and are thought to be proteinaceous iron salt minerals composed of boiling stone and montmorillonite clay minerals with high water content in the porous stone material, although there are various theories as to the origin of their composition. In Oyaishi, the miso part of the stone deteriorates easily and at a faster rate than other parts of the stone.

Due to its porous nature, it deteriorated faster when exposed to the elements in the open air. Especially after World War II, its use declined due to the spread of concrete. Especially when used on the exterior walls of private houses, it absorbs water from rain and snow, causing repeated expansion and contraction, which can lead to deterioration, blackening, and peeling.

In recent years, however, the development of thin slicing technology of about 2 cm and the appearance of the product have been re-evaluated. Furthermore, it has been found to absorb moisture, eliminate odors, and have acoustic effects, and its use in housing, store interiors, and music halls is spreading.

Use of Underground Cavities
The vast underground caverns that remain at the site of abandoned quarries are used for storing and aging wine, sake, natto (fermented soybeans), and other products. They are also used as sightseeing and learning facilities as the Otani Museum. The area is also actively used for movie locations, parties, and exhibitions for its extraordinary scenery.

Mining Method
There are two methods of mining Oya stone: “flatbed mining,” in which the stone is dug down from the surface of the earth, and “fence mining,” in which the stone is dug horizontally from a vertical shaft. Because the strata in the Oya area alternately contain layers of high-value stone and low-value stone, the fence digging method was introduced from Izu Nagaoka (present-day Izu-no-kuni City) in the late Meiji and early Taisho periods and was adopted as an epoch-making mining method.

There are two types of mining methods: open pit mining and underground mining, and underground mining is further divided into two methods: the “hara-butobo” method and the “nagakabe” method.

The Hand-Digging Era
In the pre-mechanized era, pickaxes were used for mining. In the hand-digging method, it took about 4,000 swings of the pickaxe to dig one stone of the size of 50 stones, and one person could dig about 10 stones per day. This manual mining continued until around 1960, when the mining method was mechanized.

After Mechanization
The mechanization of Oya stone excavation began in 1952, and by 1960, machines were widely used throughout the Oya region. With the mechanized mining method, one person can excavate 50 Oya stones (size: 150 x 300 x 900 mm) per day.

Transportation
In the hand-digging era, the stones had to be carried out from the quarry deep underground on one’s back. One stone (50 stones) used for a stone wall weighed about 70 kg, and it is said that one person carried out even the heaviest stone weighing 140 kg. The stones were transported by horse-drawn wagons, trolleys, and ships to distant places. Nowadays, a machine called a motor winch is used to hoist stones from deep underground quarries, and trucks and freight trains are used to transport the stones throughout the country.