The pulp manufacturer Ab Jämsänkoski made the decision to start paper manufacturing in 1897. Mechanical pulp was required in case cellulose was not always available in sufficient quantities as raw material for paper. After the company had purchased rights to the water power of the whole rapids, construction of the groundwood plant and paper mill began on the site of the present Ilveslinna. By the end of the year 1898 the buildings were complete. At first, two pulp grinders and three hollanders were installed in the groundwood plant. The mechanical pulp was used as raw material for wrapping paper and wallpaper.


In 1899, the company also acquired Rekolankoski and built a second groundwood plant, as more groundwood was needed for the planned second paper mill. The plant was built of wood, but the adjoining mill was built of brick. The mill part has been preserved and today houses a restaurant. The groundwood plant was equipped with four turbines, three grinding machines and electric lighting.

The groundwood was intended only as raw material for the company's own paper mills. The quantity of mechanical pulp produced was totally dependent on the amount of water in the river, as the grinding machines were connected to the turbines by direct power transmission. From time to time, groundwood had to be purchased from elsewhere due to lack of water. The Jämsänkoski groundwood plants were a small unit in the factory complex. In the first half of the 1910s they employed 66 workers in all. When Jämsänkoski became part of United Paper Mills Ltd in 1920, the two groundwood plants had five grinding machines between them, with production rising to 4,000 tonnes in a good year for water.

Operation of the Rekolankoski groundwood plant slowed down after the hydropower plant was completed in 1922. However, the plant operated at times during high water. Operation stopped altogether in 1928 and the wooden works building was pulled down. But the Patalankoski groundwood plant had been equipped with a new turbine and grinding machine in 1926, and its output rose to peak at 2,000 tonnes a year. During low water, the plant could also be operated on electricity generated using steam power. Concentration of paper production on fine grade papers containing chemical pulp put an end to operation of the Patalankoski groundwood plant, too, in the early 1930s. Its turbine was harnessed for power production.