| The restoration of valuable documents is a service offered by the Zentrum fuer Bucherhaltung GmbH (Center for Book Preservation) in Leipzig. Amongst other things the ZFB has become well-known for the initial treatment of the books from the Duchess Anna Amalia Library in Weimar after the fire there in September 2004.
In general, to save the valuable information in the unique historical documents, the ZFB applies many processes they have developed themselves. For example, damaged books are made as useable as possible, and brittle individual pages are given a portion of their elasticity again.
In order to develop new processes and carry out research on influences such as the effect of water additives, used to extinguish fires, on the aging behavior of paper, two strength measurements are implemented:
> measurement of the tensile strength of paper in accordance with ISO 1924
> zero-span breaking strength measurement in accordance with TAPPI T 231
In general, two machines are required: Universal testing machines measure the tensile strength in accordance with ISO, whilst the TAPPI test requires a single purpose machine. The ZFB was faced with the task of cost-effectively performing both tests on one universal testing machine. Extensive analysis beforehand by the ZBF demonstrated that tensile test results from a Zwick testing machine were far more reproducible than a machine from another manufacturer. The ZFB therefore asked Zwick to equip the Zwick testing machine for the zero-span test as well.
The technical challenges in implementing this were:
> reproducibility in positioning the grips in very close proximity to one another
> aligning the grips very precisely to one another
> Developing and manufacturing the appropriate jaws
Since Zwick testing machines have a numerically controlled drive system the positioning precision was already available in the inherent machine design. The exact alignment of the grips to one another could also be solved by using components from composite testing applications. Only the jaws had to be specifically designed for the requirements of the zero-span strength test. After a test phase and optimization the experimental design operated as expected for the tensile test as well as the zero-span test. The ZFB therefore released the Zwick machine for series testing and has already performed more than 15,000 individual tests in the first four months.
The ZFB recognized additional useful factors from the results of the optimization of the grip geometry and clamping force:
> The testing machine measures strengths in the zero-span test which are less dependent on clamping strength than for a single-purpose machine. To a great extent this dependence can be neglected, and eliminates the need for multiple measurements with different clamping strengths. Not only does this save time, it actually makes testing with historical paper specimens possible.
> The grip separation is infinitely variable down to 5 µm without any problems whilst the optimum performance is achieved with a separation of 25 µm. In contrast, the single-purpose machine only allows grip separations down to 0.1 mm and in steps up to 0.6mm. Since the strength in the zero-span strength test decreases significantly between a grip separation of 0.1 mm and 0.5 mm, grip separations greater than 0.1 mm should be used with care.
> In contrast to the single-purpose tester with its circular grip movement, the precise linear movement of the jaw in the Zwick testing machine allows paper with grammage up to 100 g/m2 to be tested.
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