Although the supply of natural stone within the borders of the Netherlands is rather limited, natural stone has been used as a dimension stone in almost all important Dutch monuments and many others.

Bog iron ore and northern erratics have locally, been used, and in the southeastern province of Limburg, the local Carboniferous sandstone ('kolenzandsteen') and Maastrichtien limestone ('mergel') have been used. Most of the natural stone applied, however, was (and still is) imported from abroad. The type of stone imported occasionally exerted an important influence on Dutch architecture, with as most evident example the 13th century Scheldt Gothic style in the Zeeland province, which style was determined by the use of limestone from the Tournai (Doornik) area in Belgium.

Tuff deposited by volcanoes in the Eifel region, Germany, was introduced in the Netherlands by the Romans, and recycled during early medieval times, when also new quarry material was important. During the latter period, trachyte from the Drachenfels in the Siebengebirge, north of the Eifel, was also used, especially in Dutch towns belonging to the German Hanse league. Because of toll levied on the river Rhine, both Römer tuff and Drachenfels were pushed out of the market by locally produced fired clay brick and imported sandstone like the Bentheim, Obernkirchen and Red Weser or Bremer (= Buntsandstein) sandstone from the 15th century onward, which could be shipped toll free. Later, tuff from the Eifel region, Weiberner this time, was used again together with sandstone. In the south and west of the country, Eocene sandy limestones from Flanders (Gobertange, Lede) were elaborately used since 14th century, later also being replaced by sandstone (which in the west of the country was introduced from Malines (Mechelen), in Flanders!). Devonian and Carboniferous limestones from the Ourthe and Meuse Valleys in Belgium have also widely been used in the Netherlands ('hardsteen', 'Naamse steen'). During 19th and 20th Century restorations, many new types of natural stone were introduced, like southern German and Scandinavian granites, many French limestones, Ettringer tuff from the Eifel, Italian tuff (peperino), etc.

Obviously, the mineralogical and microstructural characteristics of the stone are important factors for weathering and durability, together with building details and exposure conditions. Mechanisms of decay cannot, however, be explained by knowledge of these characteristics, as this knowledge is far from complete. Over the years, TNO Building & Construction Research, Delft, and TNO-NITG, Netherlands Institute of Applied Geosciences, Utrecht, have been involved ill research on natural stone in many Dutch monuments of (inter)national importance, like, the St. Servaas church (Maastricht), the Our Lady church (Breda), St. John's cathedral ('s Hertogenbosch), the National Monument (Amsterdam), the Royal Palace (Amsterdam) and Peter's church (Leiden). In the present special issue of Heron, a selection of current research on natural stone is presented.

The papers cover the weathering and durability of some of the most widespread natural stones, viz. limestone (Dubelaar et al., Larbi et al.), tuff (Nijland et al., Van Hees et al.), sandstone (Nijland et al.), as well as specific issues, viz. the role of stylolites (Larbi) and laser cleaning of sandstone (Nijland & Wijffels). Most of this research was commissioned by national and local authorities and owners wanting to preserve and conserve objects in natural stone, or architects working at restoration projects. We are grateful for both their support and for the opportunity to present this special issue on natural stone.

Timo G. Nijland & Rob P.J. van Hees
TNO Building & Construction Research, Delft