In an unusual combination of historic building techniques and the latest in 21st century bonding technology, Araldite adhesives are being used in the restoration of Borgholm Castle in Sweden.

Built in the 12th century, the castle survived intact for 700 years but fell into disrepair in the early 1800s and was eventually burnt down in 1806.

Today it is the subject of a major restoration programme, the current phase of which is the repair of one of the round towers, where a pine beam structure 14m in diameter is being built to support a limestone floor.

The new floor consists of a layer of beams containing 1,650 bolts. A layer of concrete is poured on top of the beams, a layer of insulation goes on top of that and a sealing layer caps it off. Gravel will by used to level the base to accept a limestone floor that will be laid.

"The bolts are driven 200mm into the 300mm thick beams," says site manager Lars Magnusson. "These are not screwed in, as would be the traditional approach, but secured using Araldite adhesive."

The wood being used for the beams will not be fully seasoned because of the impracticality of waiting for 300mm square timbers to dry before using them. Consequently, the wood will move as it dries out, which would gradually lead to bolts that were screwed in becoming loose and the whole structure becoming unstable. Hence the glue.

"We drill holes 22mm in diameter and the bolts are 20mm diameter," says Lars. "Before inserting the bolts we fill the holes with Araldite using a compressed air-driven handgun. Araldite is highly reliable and adheres well to both materials."

One important factor is that Araldite does not absorb moisture, which would break an adhesive down and shorten its life expectancy, especially through cold and heat cycles. Another advantage is that Araldite will adhere to wood that is not completely dry.

Once the new floor is complete, the 100,000 people who visit Borgholm Castle every year will be able to walk out on to the new terrace created in the tower and enjoy views over the Allvar and Kalmar Sound.

Araldite comes from the Huntsman Group whose UK headquarters are in Duxford, Cambridge.

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