The paper mill was once again extended in 1905. An extension 20 metres long designed by Valter Thomé was built onto the West part of the mill.

In 1911, the mill was again thoroughly renovated. A new paper machine, PM I, was bought from J. M. Voith. New pulp bleaching equipment was also bought and a new hollander room was added. All old machines were renewed. The goal was to produce better quality white and coloured fine paper. The paper machine PM II, which was rebuilt, began operating in 1913.

Valter Thomé again drew the plans for a new extension to the mill in 1914. After the renovation of the mill, it was possible to manufacture typing, printing, and wrapping paper, as well as wallpaper. Newsprint was also manufactured with PM I during the years 1919-1920.

In 1925, the manufacture of greaseproof paper began with PM I. As greaseproof proved to be in constant demand, a new machine for greaseproof paper, PM V, was ordered from J.M. Voith in 1929. A new machine hall was built for it as well. Upon order, the machine was the largest of its kind and it produced more than 20 tons of paper in 24 hours. Paper converting was also further developed.

In the 1920s and 1930s, almost all of the extensions and renovations of factory buildings were designed by architect W.G. Palmqvist.

The paper warehouse was built as soon as the 1920s. It was used to store and load paper.

At this time, the so-called dry rapids was still used to float timber between the lakes Keurusselkä and Kuorevesi. It was still an open float-way. In connection with the extension of PM 5, it was left under the paper mill and was turned into a tunnel for floating timber.

At the turn of the century, the company built a new sauna on the factory grounds. It was built according to plans made by the architects Lindhal & Thomé. The sauna included a separate section for gentlefolk, two laundries and a bakery.

All the inhabitants of Mänttä could use the sauna for the same fee, whether they were employed by the company or not. However, one big change in the sauna culture took place at this period. When the cellulose mill was being built in 1912 and a great number of construction workers came from outside Mänttä, it was decided that the tradition of women and men going to the sauna at the same time should end.

At a later stage, the workshop laundry was situated in the sauna building. The building was torn down in 1950.

The main warehouse, built in the 1890s, was still in use. The building was torn down when the paper mill expanded in 1967.

The buildings of the Mänttä engineering workshop, designed by architect W.G. Palmqvist, were completed in the autumn of 1921. Work began in the beginning of the next year. During the first years, old equipment, which had been in use at the Sassi forge and elsewhere, were used in the engineering workshop. The equipment was used to build machines for the factories belonging to the company and for various kinds of repair work. The company’s carpenter shop was situated on the second floor of the building until it was moved to Kolho in 1928. The manufacture of pumps began after an initiative in 1923 set by the technical manager of the engineering workshop, engineer Emil Oesch. Gradually, the Mänttä engineering workshop specialised almost entirely in the manufacture of various types of pumps and fire-fighting equipment. The first actual fire engine in Finland was built at the Mänttä workshop in 1925.

During the recession of the 1930s, production at the workshop was reduced. By the end of the decade, the manufacture of pumps for fire-fighting had come to an end and operations concentrated on producing flat sifters and pumps, among other things. The workshop produced ammunition for the armament industry, including grenade shells, between the years 1939 and 1944. After the war, a major part of production was exported to the Soviet Union as a part of the war reparations. Tn the beginning of the 1980s, the old workshop building was torn down when the factory was extended and the company concentrated on the production of paper.

The workers of the engineering workshop and the paper mill made their way to and from work through this gate. The way in was on the right side and the way out on the left side. Many workers spent time chatting inside the hut before and after their shift.

The cellulose mill gate, designed by Jarl Eklund, was at first situated right next to the steam power station. After the power plant was expanded, the gate was moved farther away from the factory.

A pass had to be presented each time a worker went through the gate and once a young guard at the gate, who did not yet know the managers of the factory by sight at the time, did not let Chief Engineer Silfversparre through the gate. The Chief Engineer got himself a pass the following day and sewed it into the lining of his hat. From then on he always showed the young guard his pass by raising his hat - he never did this with the other guards.

The head office of G.A. Serlachius Oy was completed in 1934. It was designed by architects Jung & Jung. In the beginning, almost all the company’s office work could be done in this building.

Being an art lover, Gösta Serlachius placed many works of art into his head office. The main entrance hall of the offices was decorated with the large frescoes "Country" and "Forest" by Lennart Segerstråle, and by a frieze depicting the birth of the Mänttä mill. The sculptures "Mother's pride" by Jussi Mäntynen were situated on both sides of the main entrance. A fountain with a sculpture of a boy and a fish by Viktor Jansson was placed in front of the building. An art collection with motifs from the wood industry was placed in the offices of the Director and the Board of Directors, it contained works by Hannes Autere among others.

The building called Plevna had been completed in the 1880s. It was still used to house workers and their families. The building was torn down in 1954.

The building was completed in 1906. The only industrial enterprise in Mänttä at that time having no connection with the activities of G.A. Serlachius Oy operated in the building until 1918. The shirt and apron factory, owned by businessman Kaarlo Heikkilä, normally employed around 40 seamstresses. The factory produced shirts, aprons, and underclothes. A small bookshop, the first in town, was also situated downstairs.

Heikkilä transferred his production to Tampere when the Commissary General of the entire white army, Gösta Serlachius, ordered a large number of summer uniforms from the Heikkilä factory in 1918. The newly formed company was called Heikkilä & Kestilä Oy, and it became one of the largest clothing factories in Finland at that time.

The former factory building in Mänttä was torn down in 1968.

Built in 1800, Törmälä was still in use as a residential building. It was torn down in 1966.

This residential building for workers was built in 1908. It was built near the exact site of the old main building of the Koskela estate, and was named after it.

The building was torn down in 1968.

Varala was built around 1910 to house workers. During the Finnish Civil War, the building was used as the headquarters for the Mänttä Civil Guard. Prisoners were also kept there. The company kept its canteen in the building during the 1920s.

The building was completely renovated in 1940.

In 1928 G.A. Serlachius Oy donated the church, designed by architect W.G. Palmqvist, to the parish of Mänttä. It was presented to the parish in connection with the company’s 60th anniversary.

Renowned Finnish artists took part in the furnishings of the church. The altarpiece and the stained glasses beside the altar were designed by Alvar Cavén. The round stained glass in the organ gallery was designed by Eric O.W. Ehrström. All the wood carvings are made by artist Hannes Autere from Saarijärvi.

The stables, completed in 1883, were still in use. As late as 1947, the construction department had five horses, while three others were reserved for various transportation tasks, others were used by the company doctor, the head office, and the welfare department. The company used horses until the 1950s.

By 1950 the stables were in poor condition and they were torn down.

At the turn of the 1920s and 1930s, the G.A. Serlachius Oy’s transport department was trusted to operate the company’s lorries. The first lorries owned by the company were purchased in 1929, 1931 and 1933. The garage, designed by W.G. Palmqvist, was built on the factory grounds in 1928.

The building was torn down in 1965.

Architect Valter Thomé designed the Mänttä fire station which was completed in 1905.

The Mänttä mill’s voluntary fire brigade also took care of putting out fires and performing fire inspections for the entire municipality in the 1920s and 1930s.

The fire station was torn down in 1961. A new station was built farther away.

In 1930, G.A. Serlachius Oy purchased the last shares of the Mänttä rapids owned by private citizens. Only after this was it possible to utilise the power of the rapids to its full extent.

The construction of the new power plant, drawn up by W.G. Palmqvist, began in 1931. The groundwood mill, located on the island in the river, was torn down to make space for the new building. The rapids were straightened out, the channels on the east side were blocked and the west channel was made deeper and more narrow. New bridges were built on the shores of Koskelanlampi and Lake Kuorevesi.

The hydroelectric power plant was completed in 1933. The building has seen no significant changes since it was built.

The Lotta cafeteria, designed by architect Jarl Eklund, which was later called the Martta cafeteria, was completed in 1941. The local chapter of the Lotta Svärd organisation began in 1920, and it continued serving in the cafeteria until 1944 when the organisation was disbanded according to the truce agreement between Finland and the Soviet Union. After this, the Martta organisation served in the cafeteria.

The building was torn down in 1959 to make space for a building that would house a water purification plant and offices.

Erected in 1921 and designed by architect W.G. Palmqvist, this memorial was made to commemorate commercial counsellor G.A. Serlachius.

In 1897 Emil Wikström made a bronze portrait of G.A. Serlachius, a new casting of which was placed in the middle of the memorial. Two large male figures representing industry and agriculture stand guard on both sides of G.A. Serlachius.

A bleaching method using liquid chlorine was developed in the United States in the early 1930s. The Jämsänkoski mill was the first in Europe to take the method into use in 1934. Other Finnish paper mills soon followed their example.

The bleaching plant, designed by Jarl Eklund, began operations in 1937. Prior to that, the mill had had a couple of bleaching hollanders used for small batches. Gösta Serlachius himself had an influence on the architecture of the bleaching plant. It was modelled after a picture of an industrial building he had seen in the "Skogen" magazine.

Pulp bleaching began in 1938, after which about half of the pulp was bleached.

The Mänttä castle had to be torn down to make room for the bleaching plant which was built right next to it.

The pulp warehouse, designed by architect W.G. Palmqvist, was completed in the beginning of the 1930s.

The cellulose mill building, designed by architect Valter Thomé, was completed in 1914. Engineer A. M. Hedbäck, together with specialists Eber and Arvid Schmidt, were responsible for the technical planning. Future extensions were considered also during the planning stage. At first, the building housed two digesters. The walls were made of bricks, which were laid upon reinforced concrete pillars, and the ceilings were also made of reinforced concrete to ensure the building would stand strong. The machinery was the best possible of that time. The plan was for the plant to produce 6000-8000 tons of sulphite cellulose per year.

Wood chips were brought from a hopper to the cellulose mill with a conveyor and dropped into the digesters through funnels on the top floor of the digester house. Digestion time was c. 8-9 hours. After digestion, the pulp was flushed from the digesters down into underground tanks. From here, the pulp was conveyed to the washing plant where splinters and unbroken fibres were cleaned out.

Part of the pulp was pumped wet and unbleached straight to the paper machines. Around half of it was dried and made into bales. The mill was extended at a rapid pace. A third digester and pulp tank were purchased in 1924, the fourth and fifth digesters were installed in 1935, and old digesters were overhauled in 1937.

The first wooden acid tower was built at the same time as the cellulose mill in 1914. It was torn down in 1925 when two acid towers made of concrete were completed. The new acid plant, designed by Jarl Eklund, together with a third acid tower made of concrete were built in the 1930s.

The cooking acid used in pulp production was calcium bisulphate. First, sulphur dioxide was prepared at the acid plant either by separating it in ovens from pyrite or by burning it out of sulphur in cylindrical rotating furnaces. The sulphur dioxide was blown with a centrifugal blower into the acid towers in which it was combined with water and formed into raw acid as it dissolved the limestone in the towers. Then the raw, or tower acid, was rectified in the first acid tank using gases from the digester house. It was then conveyed via two storage tanks to a pressurised acid tank, and was pumped from there as completed cooking acid to the digester house of the cellulose mill.

The steam power station, designed by architect Valter Thomé, was built in 1913. The plant’s chimney stack was c. 60 metres high. The people in Mänttä called it the "white chimney stack". It was torn down in 1973.

A Brown & Boveri Co. turbo generator and two Babock & Wilcox boilers were installed in the steam power station. In 1925, a back-pressure turbine was installed for more power.

A thermal power station, designed by architect W.G. Palmqvist, was built in 1935 next to the power station which was designed by Thomé. Also, a new 116 m chimney stack was erected, known as the "tall chimney stack."

After a few years, the output of the plant was increased by installing a new condensing turbine.

Although the timber arriving at the Mänttä mills was already debarked in the forest to some extent and cut into one metre logs, it had to be debarked more carefully before being cut into chips and digested.

In 1914, the Valter and Ivar Thomé architect’s office designed the first debarking plant of the Mänttä mill. It contained only one debarking drum.

In 1918, due to the lack of fuel caused by the war, six Finnish sulphite cellulose mills were granted permission by the state to produce alcohol to be used as fuel for engines.

The spirits factory, designed by architect W.G. Palmqvist, was built next to the Mänttä cellulose mill during that same year. Bricks for construction were delivered by a brick factory in Monha owned by G.A. Serlachius himself. Stone was brought from the quarries owned by the company in Koskela and Isoniemi. The factory was completed right before prohibition in Finland, but it had to wait until after 1927, when permission to produce industrial and hospital spirits was granted, to begin operations. In 1932, Oy Alko Ab granted G.A. Serlachius the right to produce strong spirits.

The spirits factory produced sulphate spirits until 1955. Production was at its peak from the late 1930s to the beginning of the 1950s.

The Koskitalo house was built in the 1880s as the grounds manager's living-quarters. After the Mänttä castle was torn down, it remained as a reminder of earlier times on the isthmus. At that time, though, it was no longer in residential use.

During the Second World War, the building was used as a doctor's office. Various offices were situated in it after this, and finally, it was the office of the company’s agricultural and power departments, together with the company’s paper shop.

The building was torn down in 1961.

This building, built as soon as the 1870s', was used as living-quarters for workers. It was constructed from logs and was at this time the oldest building on the factory grounds. When new terraces for Tehtaankatu street were built, this building no longer was right on the shoreline.

The building was torn down in 1958.

The water straining house was built in the beginning of the 1900s to serve the first power plant.

The power plant took water through the strainer located in this building. In this way, all larger impurities could be removed from the water.

The old Hoffren store built in 1892 had been bought by the company and was named Kerhola. During 1940-1944 the building housed the headquarters of the Civil Guard, after which it was turned into the company’s policlinic which provided first aid for accidents at work. In 1954, the cellulose mill office moved into this building. Scouts held their meetings in the upstairs rooms.

The building was torn down in 1959.

Riimala, a workers' residential building, was built in 1895. After the cellulose mill began operations, it no longer could be used for this purpose. It then housed a maternity ward, forest department office, payroll office, the company library, and the G. A. Serlachius general vocational school for girls, maintained by the company and the local authorities together.

The building was torn down in 1950.

The company’s hospital building, completed in 1883, was still used as such. The hospital was closed when the Mänttä general hospital began operations in 1953.

Toivola, completed in 1883, housed a hospital for infectious diseases up until 1953 when the Mänttä general hospital was completed. The building was then turned into a clubhouse for the supervisors and middle management.

The building was torn down in 1983.

Used to store the pyrite needed by the acid plant.

This building, designed by the Valter & Ivar Thomé architects office, was completed in 1916. At first it was used as a greenhouse, but was turned into a dairy during the late 1920s.

All the gardening activities of the company were located here. A garden shop was located in one end of the building for a long time even after the dairy had began its operations.

The building was torn down in 1954.

New living-quarters for the grounds manager had been built around the Mänttä estate in the beginning of the 1900s. Some of the structures of the old house had been used for its construction. A small cottage belonging to the original estate remained alongside this building.

The building received a direct hit when the bombers of the Soviet Union attacked Mänttä on January 20, 1940.

There are two Asemankulma kiosks located on Tehtaankatu. The Kuitunen kiosk, located near the dairy, was known as ’Miljoonahuiske’ (‘one million swishes’) to the local people. The kiosk mainly sold beer. The Rautatiekirjakauppa kiosk (railway book shop) was located near Kiusala.

The building was completed around 1920 as the home of the company gardener. It was torn down in 1956.

The Tarhala outbuilding was situated to the south.

As the factory area experienced various changes, the main east to west traffic route in the area between the rapids was moved right next to Koskelanlampi. The old main road in the middle of the isthmus was left inside the expanding factory grounds.

During the 1920s, G.A. Serlachius Oy suggested that the normal gauge railway replace Mänttä’s old narrow-gauge railway. A decision was made in 1927 in favour of Serlachius, and the new line was opened for traffic in 1929. This meant a great deal to the company. The vast amount of labour needed for loading and unloading at the Vilppula station was no longer needed. All loading could now be carried out at the factory.

Although the narrow gauge railway between Mänttä and Vilppula was torn down when the mainline railway was completed in 1929, a small network of narrow-gauge track remained for a long time inside the factory area. It was used for internal transports.

The Mäntän kirjapaino Oy printing house still operated in the former Färm shop built in the late 1870s.

The building was torn down in 1961.