Lead sheets for organ pipes - Positron Spectroscopy for cultural heritage

Lead sheets are a basic material to build metal pipes for organ pipes. Mostly lead gets alloyed with tin and in this manner its properties have been seen superior over centuries. But, the classical organ construction (16.-18.century) in the northern parts of Europe comprised the use of alloys with very low (2-3%) or no tin content because of cost reasons. In such cases the lead material shows critical properties regarding the mechanical strength and density. Nowadays, the restoration of organ instruments from this period requires the production of new material to replace corroded material or to substitute missing pipes or other organ parts. An important issue is the engineering of such material by mechanical hammering to increase the density. Microscopic cavities inside the casted material get closed in this way and the mechanical strength gets increased. Up to now there exists only an empirical knowledge about the quality changes due to the hammering treatment within a period of several centuries.
We used the Gamma-induced Positron Spectroscopy (GiPS) setup at ELBE at Helmholtz-Zentrum Dresden-Rossendorf to investigate the porosity of hammered and non-hammered material both for historical material from the 17th century and for newly produced sheet material for the first time to get microscopic information about the effect of the hammering practise. The Gamma-induced Positron Spectroscopy setup at ELBE is worldwide the only suitable setup to investigate solids with an information coming from the whole volume. Results obtained from this are compared with conventional depth resolving Doppler Broadening Spectroscopy measurements using a slow positron beam.