medieval-religion: Scholarly discussions of medieval religion and culture

Hi Maddy,

Nice technical story, but it's not all that can and should be said about
painting on stone (or brick). Medieval man was not stupid: he knew that the
weather (wet) could influence the wall on which he was painting, so he took
precautions. Depending on the prevalent winds in an area there were several
things one could do.  Therefore gutters and drains were applied to lead away
water from a wall. They also knew (and this is all over Europe!) how to
insulate a wall, waterproof is as it were. They used tar for that or made
sure the plaster consisted of many layers, from course to fine, the better
to fuse with eachother and the paint. The main thing however was to prevent
water accumulating on top of or at the foot of a wall, so it could not
penetrate the stone and attack the paint-layer from behind. You probably
know that a fresco, which is a specific technique of wall-painting (mainly
southern European and imported into the northwest at the end of the Middle
Ages), cannot be harmed by water from the front, but when a wall gets wet it
causes salty blooms to appear in certain colours and that destroys these
paintings. Fresco painting is a technique that uses the chemical fusing of
wet lime plaster and pigments and is especially fitted for warm countries.
In colder climes this does not work so well, so here the technique called
secco is used. This consists of painting in a tempera way on dried plaster.
The pigments in this technique are mixed with egg (whites, yolks or both).
The painted layer gets very hard after it has dried.

Some remarks. It was very rare to work with linseed oil on walls. See what
happened to Leonardo's Last Supper in Milan. It can only be done on inner
walls and even there it is not stable.

I have heard of sealing walls with egg white but not of doing it with
tallow. You'd need al lot of eggs to seal a church but it seems to have been
done, if only in parts. Cennini mentions that for oil painting on walls (as
is done in Germany, he says) you need a plastered wall that is sized with
egg glaze. Oil painting in C's day was not the kind we have today (alla
prima), but consisted of (many) layers of tempera paints and final glazes of
oil colours over that. Tallow, like wax, is even more unstable as a ground
than oil on stone.

Plantbased dyes can be used to make pigments, but they need a carrier to
work, because they are not solid but watery.  Saffron is about the only
exception as this is a very strong colour. It is and was also very expensive
and you need a lot to paint f.i. a foot square piece of wall. I have never
heard it used for wall painting, but you can find it in some miniatures.
There were other yellows that were used on walls, mainly ochres, and they
were very much cheaper. So why would they use saffron? Other vegetable dyes
may be combined with acids or bases to form lake-colours, but these weren't
used on walls as well. Too instable and likely to flake in time.

Wall colours were minerals like the ochres (including yellow, red and pink,
sienna, umber and terre verte) and metals or metalcompounds (lead, minium,
cinnabar) and copperores like azurite, malachite, plus soot of several
types. Most of these colours are pretty stable and do not lighten easily. I
agree that leadwhite does destroy some other colours after some time and the
lead in minium tends to darken the orangy red. But the better painters knew
that and usually closed them off by 'wrapping' them in egg-white, which
dries up very hard, almost glass like.

Gesso, as I said before, is a mix of a chalk with glue, and does work best
on wood. If used on stone, it has to have a plaster underlayer as contact
with the stone will make the glue react, depending on the type of stone.

Sunlight may lighten some paints earlier than others through the centuries,
especially greens and blues, but I think that the light in all but the
lightest late-gothic churches (with no stained glass windows: so this was
mostly after the Reformation demolishings) does not really fade the colours
that much. Remember that most medieval wallpaintings we see nowadays have
been hidden beneath layers of whitewash for centuries, which also has had
its effect on the fading. Inner south walls of churches would have stayed
bright then, because they do net get any direct light, but this isn't the
case. Fading has usually other causes. 

Light of lamps or candles does not have any fading effect on any but the
most unstable colours (which the above mentioned weren't) and they have to
be pretty close for that. Soot and smoke may have had a darkening effect on
church paintings, but it takes a long time (many centuries) to darken higher
up colors on church walls, vaults and ceilings. Stained glass stops most of
the fading effect of straight sunlight on colours.

 
Henk

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