The stonemason Nicole Held participated in the renovation work. Now she is standing in a high-ceilinged room of the minster's building hut, looking at a row of film canisters. "They're full of water with a bit of powdered stone in it," she explains. This material was removed from the stones with a drill three millimeters in diameter at depths of two and four centimeters. That's because acid rain deposits salts in the outer layers of the stones.

Nicole Held uses the Merck product Microquant® to measure how badly the salts from the sulfuric acid have already damaged the stone. To do this, she takes some water from one of the canisters, subjects it to a number of processing steps at a temperature of exactly 40°C, and notes its change of color. Then she goes to the window and turns the color comparison wheel until she finds a color that precisely matches that of the water containing the sulfuric acid. "We then base our further desalination measures on this measurement value," explains Ingrid Helm-Rommel as we ascend along the outside of the church in a rattling, jolting elevator and come to a halt at a height of 30 meters.

We're greeted by Roman Koch, who takes off his respiratory protection mask and calls out, "Welcome!" Koch is a stonemason who previously worked on restoring the Zwinger in Dresden and has been working in Ulm for the past four years. He is using a wooden mallet to rhythmically hammer on a chisel, in this case a wooden wedge.

With a few final blows he frees the stone from the "compress," a mixture of wood pulp and clay minerals that has been spread on it. In this procedure, the stone is first sprayed with water and then spread with this mixture in varying thicknesses — up to three centimeters — depending on the amount of salt in the stone. The process is repeated between three and five times. "In just over a week, the salt ions migrate into the compress as it hardens," Koch tells us, raising his voice so that he can be heard above the whistling wind.

After that, the compress is knocked off. A total of 58 tons of this material was used just for the three sets of compresses that were needed to desalinate the 779 square meters of the surface of the southern choir tower's spire. A thin separating layer of Japan paper between the stone and the compress makes the work a bit easier. During the process the stone being treated is repeatedly tested for its salt content.

"At the end of the process, the average salt content is somewhere between zero and 0.05 percent of the stone's weight, whereas we started at about 0.25 to 0.375 percent," explains Ingrid Helm-Rommel as we descend to the foot of the tower, enjoying the changing views of this city on the Danube River. When its citizens began to build their minster more than six centuries ago, they numbered only about 10,000. After the iconoclasm of the Reformation, the construction work was broken off in 1543 and not resumed until 1844. As a result, the project begun in 1377 was not completed until 1890.

Safeguarding and preserving this notable sacred building will be the job of the minster's master builders in the centuries to come as well. A stone tablet in the foyer of the church will eventually remind visitors that Ingrid Helm-Rommel was the first woman to hold this position — just like the stone tablet that now commemorates her predecessors and those that will one day honor her successors.