Salt crystallization on the wall paintings of the chancel at St. Johannes in Neustadt/W, Ortsteil Mußbach

Author: Steffen Laue
English version by Sandra Leithäuser

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The Choir of St. Johns Church, Neustadt/W.- Mußbach

The late Gothic church of St. John at Mußbach was built in 1370. At the beginning of the 18th century, a division into Catholic and Protestant areas was made. A wall in the choir arch divides the Catholic choir from the Protestant nave. While the ancient nave is still used as place of worship by the Protestants, the Catholic community obtained a new church building in 1959. Since this time, the ancient choir of St. John has not been in use. At the end of the nineteen sixties, gothic but also younger wall paintings were uncovered and are subjected to weathering ever since.

Salts and indoor air-climate

One of the damage patterns occurring cyclically is salt efflorescence of potassium nitrate (KNO₃) at up to 1.5 m height in the eastern part of the choir. This periodical efflorescence was observed over a period of 3 years. In winter (November and partially in February) new salt efflorescence was mapped in different but specified reference areas. The indoor climate in the choir of St. Johannes mirrors the exterior climate in a subdued form. The relative humidity varies all year, the air temperature in summer can sometimes exceed 20 °C and reach values below freezing in winter. No correlation between the efflorescence and relative humidity could be determined. However, the crystallization phases, noticeably, always took place after an exterior temperature drop, followed by a temperature drop in the choir. For example, at the beginning of February 1993 the outside temperature fell from +8°C to -5°C within a few days and the indoor temperature during the same period from 7°C to 2°C. At the same time the relative humidity outside varied between 80% and 90%, while the relative humidity inside varied between 60% and 75%.

When interpreting climate data in conjunction with the observation of efflorescence, the solubility properties of potassium nitrate (KNO₃) have to be taken into account: with reference to Stephen et al. . [Stephen.etal:1963]Title: Solubility of sodium and potassium nitrate
Author: Stephen, H.; Stephen, T.
) 24 weight-% of KNO₃ dissolves at 20°C in aqueous solution, while only 11,7% weight-% of KNO₃ dissolves at 0°C, i.e., less than half the amount. The lower the temperature of the system KNO₃ - H₂O, the greater the likelihood of supersaturation of a KNO₃ rich solution, resulting in the efflorescence of potassium nitrate.

The measures and observations lead to the following model for the cyclic efflorescence of potassium nitrate in Mußbach: During the summer months a KNO₃ containing solution forms in the pores of the lower part of the choir’s plinth. When temperatures fall below 10°C the conditions are right for potassium nitrate whiskers and fluff to form, because its solubility is reduced and supersaturation takes place. With the lowering of the temperature during the crystallization, a slight reduction of relative humidity around 10-20% was determined. Probably both climate factors- the lowering of temperature and relative humidity- have contributed to the supersaturation of the wall’s solution with potassium nitrate ions. The efflorescence will last until the supersaturation of ions in the solution is ended. A possible explanation for the ending of the crystallization phase is the increase of the measured temperature in the room. Through the temperature increase, the saturation limit of KNO₃ in the solution within the walls is increased. Efflorescence is halted, because the solution cannot carry more salt. Another possible explanation for the termination of the crystallization phase is, that an increase in moisture supply from the substrate dilutes the salt solution, which stalls crystal growth.

Literature

[Stephen.etal:1963]

Stephen, H.; Stephen, T. (1963): Solubility of sodium and potassium nitrate. In:: Solubilities of inorganic and organic compounds, , 180.

crystallization of a dissolved salt