THE TREATMENT OF WEATHERED GLOBIGERINA LIMESTONE: THE
SURFACE CONVERSION OF CALCIUM CARBONATE TO CALCIUM
OXALATE
Tabitha Mifsud
Dissertation submitted to the Institute for Masonry Construction and Research
in partial fulfillment of the requirements for the Degree of Master of Science in
Conservation Technology for Masonry Buildings
ABSTRACT
This work concerns the investigation of ammonium oxalate treatment on weathered
Globigerina Limestone of the “franka” type. To this end, “franka” Limestone samples
were subjected to a 5% ammonium oxalate poultice for 5 hours, after which testing
took place on both treated and untreated samples. The range of samples included fresh
quarry samples, artificially weathered quarry samples and naturally weathered
samples. From each group, both desalinated and non-desalinated samples were
studied. The non-desalinated samples included sodium chloride as the predominant
salt since the presence of this salt in Globigerina Limestone is the most representative
of the local scenario. The first part of the investigations included X-Ray Diffraction
where the formation of calcium oxalate following ammonium oxalate treatment was
verified. It was confirmed that the presence of sodium chloride did not hinder the
formation of calcium oxalate in any way. In fact, the non-desalinated samples were
found to produce more calcium oxalate than the desalinated samples of the same type.
This was attributed to the increased surface area available for reaction with
ammonium oxalate in the non-desalinated samples. It was consequently concluded
that desalination prior to ammonium oxalate treatment is not imperative. Scanning
Electron Microscopy was then used in order to look at the surface morphology which
was noted to be more compact in the treated samples. The hardness was then
evaluated utilising the Mohs hardness test which in all cases revealed a surface
hardness of 4 – equal to the hardness of calcium oxalate - in all of the treated samples.
The surface cohesion of the samples during a tape test, where tape was applied and
then removed together with any superficial surface material from the surface of the
stone, resulted in more material being removed in the untreated samples than in the
treated ones for both the desalinated and non-desalinated samples. The possible
protection from acid attack as a result of the treatment was examined through acid
resistance tests which included 4 different acids – acetic acid (5%), acetic acid (10%),
hydrochloric acid (0.5%) and hydrochloric acid (2%). In all cases, the treated samples
lost less mass when placed in direct contact with the acids than the untreated samples.
In addition, it was observed that during contact with the 5% acetic acid solution, the
treated samples underwent “whitening” in colour on the treated faces. This was then
used to look at the depth of calcium oxalate formed in the treated samples through
total immersion in a 5% acetic acid solution. Any resistance to salt crystallisation that
may have been gained due to ammonium oxalate treatment was looked at through
artificial weathering using sodium sulphate. A small increase in resistance was
registered in all of the treated samples. The water absorption properties were
subsequently examined through water absorption test through capillarity. These tests
revealed that the treatment did not alter the mode of water absorption through
capillarity. Finally, direct comparison between treated and untreated samples within
each group and to a Munsell colour chart confirmed that no colour alterations took
place following ammonium oxalate treatment. These promising results represent
ammonium oxalate treatment on “franka” Globigerina Limestone in a positive light.
In practical terms, it was concluded that this treatment could be used in the filed of
conservation as a protective treatment, possibly with some consolidating properties,
on historic buildings and monuments in the Maltese Islands.