Aesthetics and functionality in glazing for historical buildings

Preservation efforts are primarily orientated towards maintaining historical structures and, if possible, cautious restoration. However, when material loss or irreparable damage threatens to halt the effort, alternative options need to be considered explains Ulrich Huber, Sales Manager EMEA, SCHOTT AG.

The same is true when the preservation of an entire building, which – from an economic point of view – can only be achieved by converting it for a use other than its original purpose. In this process, driven primarily by economic considerations, building owners and occupants formulate the general requirements for construction components, as is the case with glazing: winter and summer heat protection, safety aspects, and photometric parameters, to name just a few. When taking historic preservation aspects into consideration, not only are the new material’s optical appearance and similar characteristics to the original material important, but also the use of an authentic production process, which, at any rate, must be based on historic technologies.

Historical Context of Glass Making

It was not until the early 20th Century, with the rolling table process, that the manufacturing of flat glass could be accomplished in a continuing process. In 1902, Belgian Emile Fourcault (1862–1919) patented a process for manufacturing flat glass utilising drawing nozzles and a vertical drawing shaft. By 1904, he had achieved technical implementation of the process. The first industrial plants were launched in spring 1914 when eight casting machines were commissioned in Dampremy, Belgium. In the 1920s the Fourcault process became the first fully mechanised process for the continual manufacture of sheet glass and was gradually implemented globally. In Germany, due to provisions in the Treaty of Versailles, the Fourcault process could not be implemented for manufacturing until 1925. The first German plant locations were in Witten (North Rhine – Westphalia) and Torgau (Saxony) where production began in 1925 and continued until 1990. Thus, machine-drawn glass was the prevailing material for window panes and facade glazing for the construction era of 1920 to 1960 and to some extent beyond that. The use of the Fourcault process heavily declined in the 1960s with the implementation of the float process.

Characteristics of machine-drawn glass

Drawing the molten glass through the Fourcault nozzle produces more or less pronounced draw marks in the final product. Thickness variations within the defined tolerance are what give machine-drawn glass its distinctive characteristic. Thus, the manufacturing process can result in certain warping. In contrast to float glass, machine-drawn glass possesses a higher plies difference. Features stemming from the smelting phase, including blisters, knots or tiny pebbles, are acceptable as long as their size and frequency do not exceed the specifications within the predetermined values. All of these characteristics give machine-drawn glass its distinctive appearance and make for an authentic production process. In its top view (reflection) uneven surfaces can be seen in machine-drawn glass.

When looking through the glass (transmission), straight lines often appear wavy (Image 1). In contrast, float glass, due to its plano-parallel surfaces, gives optically undistorted impressions in its reflection as well as its transmission (Image 2).The features of machine-drawn glass can be influenced by specific product technologies. Thus, for structures of different design eras, several types of restoration glass are available to match the original glass used on historic buildings.

These include, for example, 4.5 mm thick, sturdy, colourless glass with irregular surfaces full of character, which is also suitable for use as outdoor glazing and restoration glass and that resembles window panes manufactured at the turn of the 20th century. A minimal thickness of 2.75 mm ensures that it can be easily installed in historic window frames and window profiles. Or for a more lightly structured surface version glass, resembling hand-blown glass, or in contrast, a glass with a more dominant structure, are also available. Whereas for Bauhaus style buildings, a 4-mm thick restoration glass with a slightly irregular surface to form harmony with buildings of Classical Modernism is ideal.

Depending on the glass static necessities of the project, such as solar protection coatings, this glass can be produced with a 6 mm thickness and up to 3,000 mm in length.

Machine-drawn glass is generally low iron glass. Thus, as monolithic glass, it has a colour rendition index of 100 and it is ideal for glazing in museums, which present increased lighting technology challenges.

Additional process possibilities

In May 2012 the Deutsche Institut für Bautechnik (DIBt) – or the German Institute for Civil Engineering – issued the European Technical Approval, ETA-12/0159, for glass made using the Fourcault process. This means that these glass types are considered authorised construction products which can be used within the scope of an approved construction project.

In general, all restoration glass can be processed as insulation glass. When it comes to thermal restorations in historical buildings, insulation glass with a low overall thickness is often requested so that it can be integrated into frames worth preserving provided that they have the required load capacity and appropriate seam width. Restoration glass with a thickness of at least 2.75 mm makes it possible to have an insulated glass structure with an overall thickness of approximately 10 mm. The weight of units with a second pane made of 3 mm float glass is roughly 15 kg/m². When krypton is used as a gas filling for the space between the panes and a heat protective coating at Level 3 is applied, a glazing U-value of 1.9 W/m²K can be achieved. A selection of restoration glass types can be used if a solar protection coating at Level 2 is mandated in a refurbishing project. This is especially true when it comes to providing protection from the summer heat in buildings with large-area glazing from the Bauhaus era as well as for modern ones.

The processing of laminated glass using PVB films allows for a glazed/glass structure in accordance with a resistance class per EN 356. When it comes to user-specific lighting technology requirements, transmission values of specific spectrums for solar radiation can be adjusted. Thus, it is possible to reduce the transmission of light in the UV range of 280nm to 420nm from normally 57 per cent to 8 per cent – an important aspect to prevent the damage of assorted materials caused by solar radiation. Thermal tempering with the appropriate machine-drawn glass increases their mechanical and thermal capacities and produces fracture patterns similar to tempered safety glass.