Consolidation of stone

Jim Mann <jmann [at] amdel__com> writes

>I am looking for guidance on a method of determining the depth of
>penetration of consolidants into building stone (on site) without
>causing damage such as by coring.

We did some work about 10 years ago on the problem of
non-destructive determination of the penetration depth of
consolidants in stone. The technique used was infrared thermography
in the 8-10 micrometer band range and the results of our feasibility
study were presented at the 8th International Conference on Thermal
Engineering and Thermography, Budapest, Hungary, June, 1993. An
abstract appeared on pages 41-42 of the Abstract volume.

The method is based on the difference between the thermal
diffusivities of consolidated and unconsolidated stone. Here is a
short description of the experiment we performed. Sandstone cores of
differing lengths from 6-36 mm were consolidated uniformly and
inserted into closely fitting holes in large sandstone blocks with
the core surfaces flush with the tops of the blocks. The surface of
each block, representing the surface of an insitu wall, was warmed
uniformly by a bank of flood lamps and the temperature difference
between the consolidated core and the unconsolidated stone
surrounding the core was monitored by the IR imaging system as a
function of time for about 5 minutes. The surface temperature never
exceeded 40 Celsius. On plotting a function of this temperature
difference versus time, we obtained a set of curves that showed
maxima at particular times, which were characteristic of the core
lengths (penetration depths) and the amplitudes at the maxima were
related to both the consolidant loading levels and core lengths. We
tried two different consolidants and found that the results were the
same, qualitatively, but different quantitatively, indicating
different thermal diffusivities of the two consolidated stone
samples.

This was a feasibility study and we demonstrated that the method did
indeed work but we have not done any further work on the method. The
procedure can be carried out in the field but the equipment is
costly and some calibration would be required to get quantitative
results for particular consolidant-stone systems. I think that the
method is capable of imaging a large surface and showing qualitative
variations in penetration depth, which would appear as surface
temperature fluctuations on the imager. IR imaging technology has
improved considerably since we did our work and I think 3-5
micrometer imagers would be OK and much smaller and less expensive
than the medical imager we used.

Bill Ginell
Getty Conservation Institute
310-440-6262


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                  Conservation DistList Instance 14:44
               Distributed: Wednesday, February 14, 2001
                       Message Id: cdl-14-44-006
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