External coatings on historic buildings

Orit Soffer <oritsoffer [at] justemail__net> writes

>I am having doubts regarding the correct way of applying lime-based
>coatings on historic buildings, so that the coating would resist for
>a reasonable number of years in environmental conditions which
>consist of pollution, sea air and a warm climate (no frost
>problems). ...

In Roman times Vitruvius recommends 7 coatings! no more than 4 or 5
have ever been found by archeologists. Since Renaissance times, in
Italy we usually use 3 coatings. The first, thin (about 0,5cm) with
coarse aggregate; the second, thick (2-3cm) with medium aggregate;
the third is a finishing coat (2-3mm) with fine aggregate and earth
pigments colouring.  All sorts of additives were used, sugar for
anti-frost, beer yeast and/or cheese for porosity, etc. In Venice
the famous "marmorino" (marble-like) render used Cocciopesto (ground
terra-cotta) as an ingredient. The final layer was hot-finished with
beeswax. It would be useful, but rarely done because of cost, that
the coats of render be applied over the former coat heavily
scratched to provide a mechanical key.

Waiting several months before applying a new coat is unheard of (by
me). All coats are to be applied on a damp surface. In any case, it
is not good practice to leave surfaces that require protection open
to the elements any longer than necessary; perhaps the conservators
who recommend waiting several months before applying the finish are
referring to fresco paint work were it is necessary to have high
suction for the finishing coats. As we know the maintenance costs of
lime renders was very high. With natural lime alone the carbonation
process can take several decades! at least 5 or 6.

To-day, to give some longevity to external renders the use of
hydraulic lime would be essential. The addition of some trass flour
(approximately 5% to lime by volume) would certainly provide
increased resistance to water ingress, this would minimise the
ingress of pollution and chlorides.

Alternatively, the finish coat could be a lime wash (used for
centuries in Northern Europe) especially the formulas that contain
tallow as these offer excellent external protection.

Edgardo Pinto Guerra
Westox Consultant for Europe
53040 Cetona (SI) Italy
+39 0578 238329


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                  Conservation DistList Instance 17:8
                   Distributed: Tuesday, July 1, 2003
                        Message Id: cdl-17-8-007
                                  ***