The integrity of the surface painted glazed layer has a great impact on the safeguard of the artistic glazed ceramics, also involving the transmission of their intangible aspects. Glaze delamination and spalling is a severe problem for these artworks; revealing the occurrence of early decay processes represents a highly desirable possibility helping the preventive conservation. The authors wish to explore the possible ways to pursue this objective, attempting to non-destructively detect thin glazed delaminations when they are not yet visible to the naked eye. In recent research studies, contactless vibro-acoustic methods allowed to identif y sub-surface artificial cavities, a few-hundreds microns deep, in laboratory models and natural delaminations in historical glazed ceramic tiles, by inducing the defect to vibrate by means of an external acoustic pressure field. The present study adds further insight to this issue by analysing a greater number of real tiles, mainly focusing on some apparently integer samples with respect to a delaminated one. The samples belong to a little collection of antique Portuguese azulejo dated between the 17th and the 18th century. Additionally, a modern tile is used as reference. In these laboratory tests a contactless acoustic wave source in the audio frequency interval is employed to induce the vibration in the sample, whilst a Laser Doppler Vibrometer (LDV) measures the vibration velocity of the sample's surface. The experimental data are displayed as 2D images of the vibration velocity at the most significant frequencies. The characteristic normal modes of vibration clearly characterize the undamaged tiles, with low velocity values and geometric patterns, being the sample free to vibrate as a whole according to its preferential natural modes. Elseways, confined spots with high velocity values indicate the position and the extension of glaze delaminations in the damaged tile. The frequency profiles of the most representative points provide further information: small and narrow peaks at specific frequencies in the first group of samples; broad and high velocity peaks in the second one. Laser Doppler Vibrometry is particularly powerful to evaluate the conservation state of glazed ceramic tiles due to its high spatial resolution and non-invasiveness, although an accurate device-sample alignment and the data interpretation from expert operators are important requirements. The present study examines the vibrational characteristics of glaze delaminations in a damaged tile, and compares them with those found in the undamaged tiles highlighting the main differences. The experimental results help a better understanding of the most appropriate methods, the optimal experimental settings and data analyses to adopt for approaching an ever growing effectiveness in the detection of the early delamination processes.
GLAZED CERAMIC TILES: LABORATORY TESTS USING CONTACTLESS VIBRO-ACOUSTIC DIAGNOSTIC TOOLS TO REVEAL GLAZE DELAMINATION DECAY PROCESSES
Paola Calicchia;Sara De Simone
2019
Abstract
The integrity of the surface painted glazed layer has a great impact on the safeguard of the artistic glazed ceramics, also involving the transmission of their intangible aspects. Glaze delamination and spalling is a severe problem for these artworks; revealing the occurrence of early decay processes represents a highly desirable possibility helping the preventive conservation. The authors wish to explore the possible ways to pursue this objective, attempting to non-destructively detect thin glazed delaminations when they are not yet visible to the naked eye. In recent research studies, contactless vibro-acoustic methods allowed to identif y sub-surface artificial cavities, a few-hundreds microns deep, in laboratory models and natural delaminations in historical glazed ceramic tiles, by inducing the defect to vibrate by means of an external acoustic pressure field. The present study adds further insight to this issue by analysing a greater number of real tiles, mainly focusing on some apparently integer samples with respect to a delaminated one. The samples belong to a little collection of antique Portuguese azulejo dated between the 17th and the 18th century. Additionally, a modern tile is used as reference. In these laboratory tests a contactless acoustic wave source in the audio frequency interval is employed to induce the vibration in the sample, whilst a Laser Doppler Vibrometer (LDV) measures the vibration velocity of the sample's surface. The experimental data are displayed as 2D images of the vibration velocity at the most significant frequencies. The characteristic normal modes of vibration clearly characterize the undamaged tiles, with low velocity values and geometric patterns, being the sample free to vibrate as a whole according to its preferential natural modes. Elseways, confined spots with high velocity values indicate the position and the extension of glaze delaminations in the damaged tile. The frequency profiles of the most representative points provide further information: small and narrow peaks at specific frequencies in the first group of samples; broad and high velocity peaks in the second one. Laser Doppler Vibrometry is particularly powerful to evaluate the conservation state of glazed ceramic tiles due to its high spatial resolution and non-invasiveness, although an accurate device-sample alignment and the data interpretation from expert operators are important requirements. The present study examines the vibrational characteristics of glaze delaminations in a damaged tile, and compares them with those found in the undamaged tiles highlighting the main differences. The experimental results help a better understanding of the most appropriate methods, the optimal experimental settings and data analyses to adopt for approaching an ever growing effectiveness in the detection of the early delamination processes.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
