Salt crystal art work in a rainy Passau

© Dietrich Förster, Kinsau

From block to cube to crystal

A gigantic salt crystal in Passau is a testament to the city’s past as an important station along the salt trade route. Incidentally, the work of art also shows how well PLEXIGLAS® can be bonded.

To make the salt crystal, the plastics specialists from k-tec GmbH in Austria first produced a cube-shaped block made of PLEXIGLAS®. To this end, they combined several sheets with a polymerization adhesive. What sounds like the simple combination of paper with glue is actually a complicated chemical process. But the result was worth the trouble: The transparent acrylic glass block is extremely stable, half as heavy as white glass and the seams are invisible to the naked eye.

Luminous crystal

Since 2015, the “luminous crystal” at the Ilz bridge in Passau has been attracting a lot of attention: What is normally so tiny, even on a fingertip, is displayed here in 1,000-fold magnification – a salt crystal which even glows in the dark. The artist Dietrich Förster from Apfeldorf created the attraction as a reminder of the importance of the “Golden Route”, a former salt route in the town of Passau, which passed by here in the Middle Ages. Here, the salt was transferred from water to a land route and transported on to Bohemia where it was used for the preservation of meat.

Sodium chloride crystal from acrylic glass

Initially, Förster pursued the idea of creating the 3.2-meter-high sculpture (its base included) from laminated white-glass plates. “But within a given budget, it was not possible to find a weather-resistant bond for the glass panes, which would permanently withstand the shear forces generated by the 45-degree tilt of the glass panes,” says Förster. The solution: PLEXIGLAS®. The brand acrylic glass from Evonik is easy to bond and process, is eleven times more stable and only half as heavy as white glass. “In addition, the colorless transparency of PLEXIGLAS® is better suited to give the impression of a sodium chloride crystal than white glass with its blue-green hue,” says Förster. This optical impression can be retained over many years because PLEXIGLAS® is both weather and UV resistant.

Bonding plastic without bubbles

“Our first step was to produce a block with one-meter edge length for the work of art. Because there are no prefabricated blocks of acrylic glass in this size,” explains Thilo Üblagger, Managing Director of the k-tec GmbH in Salzburg, which specializes in thermoforming. Put simply – the experts for acrylic glass parts bonded several layers of PLEXIGLAS® blocks with a thickness of 150 mm together to create a cube of one cubic meter. But even if this association is made immediately: This process has nothing to do with two paper sheets that are bonded together with glue.

Bonding PLEXIGLAS®

Evonik offers a comprehensive range of special adhesives for bonding PLEXIGLAS® and other acrylic glasses with itself or other materials. These adhesives are marketed under the ACRIFIX® brand. Discover the world of ACRIFIX®.

Adhesive for PLEXIGLAS®

This is a much more complex process in which all parameters must be correct: Starting with the clean cut of the acrylic glass parts to be connected to the respective suitable temperature to the duration of the process. “Just in order to achieve a bubble-free seam, you need a lot of prior-experience in applying the adhesive,” explains Üblagger. Connecting PLEXIGLAS® to PLEXIGLAS®, requires a special polymerization adhesive, in this case the ACRIFIX® 5R 0194 which was specially developed for connecting blocks. This is basically liquid PMMA, the chemically correct term for PLEXIGLAS®, which then forms an extremely stable and almost colorless compound with the solid parts to be bonded.

Everything on the bonding of PLEXIGLAS® with ACRIFIX® adhesives.

The adhesive has almost identical chemical properties as the blocks to be bonded, making the PLEXIGLAS® block look like it was made from one piece.

Thilo Üblagger
Managing Director of k-tec GmbH

First bond, then mill and polish

After the polymerization – the curing of the compound – the tempering is then carried out. This refers to the slow cooling of the acrylic glass which can sometimes take several days. Without this step, cracking from tension can occur because the previously supplied energy is looking for a way out. “The result is then approximately as resilient as PLEXIGLAS® itself,” says Üblagger.

Only after this procedure, k-tec GmbH began to mill the sodium chloride crystal out of the acrylic glass block. The edges, which are roughened and matte after milling, are smoothed and polished in a final step, thereby giving them a high-gloss, transparent surface which can be seen from afar at the Ilz bridge. The staircase-like structures of the work of art creates a visual link to the Golden Route. What’s more: The sculpture of modern material is the perfect connection to the past.