Science meets art: investigating pigments in art and archaeology (30 June 2011)

>>Welcome to what I gather is the last of this seasons
>>Thank you
>>[laughter] The last of this seasons set
of UCL lunchtime lectures.
I gather they will be restarting October.
But today's lecture is on pigments in art and archaeology
by Professor Robin Clark.
Now Dr. Clark has, sorry,
Professor Clark has led a long distinguished scientific career.
He was born in New Zealand and has been a companion
of the New Zealand Order of Merit since 2004.
After taking his first in Master's degrees
at Canterbury University College in Christchurch, he first came
to the UK in 1958 to undertake studies for his PHD in Chemistry
at UCL and he's remained at UCL ever since,
although he has managed to fit in lecturing
at over 350 other universities and institutions in that time
and also acting as visiting professor
to 13 other universities.
Besides all this he is a fellow
of many major institutions including the Royal Societies
both of this country and New Zealand and he served
on the councils of the Royal Society, the Royal Institution,
where he was sat to retrieve 36 years
at the University of London Senate.
He has received awards too numerous to go into.
Although he's now officially retired he has
in no way slowed down, as this lecture will show.
Professor Clark has long been interested
in molecular spectroscopy.
In 1969 he brought the first laser Raman spectrometer to UCL
and he's been heavy involved
in the development of this technique.
Raman is now something which is very widely used throughout
industry and chemistry but much more importantly
to all right minded people it's used extensively in museums.
We have had a Raman spectrometry in this museum now since 1999
and you'll see examples of some of the work that we've done
with it in the Treasure's of Heaven exhibition
which is opened at the moment in the round reading room
and also spectra in the, along with some of the Chinese Jades.
Professor Clark, who is a pioneer of the introduction
of this technique into art and archeology
and that's what he's going to be talking about to us today.
So with that I'd like to introduce Professor Clark
to talk about science meet art, investigating pigments
in art and archeology.
[applause]
>>Good afternoon, everyone.
When you look at a piece of medieval artwork such as this
with intertwined figures, great design, strikingly colored,
you always admire it as a piece of artwork.
I want to try and convince you
that it's also excellent science behind all of that
because when you think about it all the colors are pigments.
The pigments, for most periods of time,
were inorganic pigments, minerals
or synthetic inorganic materials.
And so you did need to know chemistry in order
to be an artist and the early artist didn't know a lot
of chemistry.
They knew that in many cases if you put A next
to B they would react and so they didn't do it.
So they had to know chemistry.
So I want to try and persuade you
that this is a branch of chemistry.
So what is the purpose, and pigments are the key to it,
so what are the purposes of looking
at pigments and identifying them?
Well the characterization, what pigments did an artist use?
Did he use a single one or a mixture and all sorts
of questions of that sort, questions to do
with restoration, repair of damaged areas,
have there been any changes in color with time,
anything to do with conservation.
What are the effects of heat,
light and gaseous pollutants and such like?
And the next question of authentication which is linked
into the assignment of a probable date to a work of art.
So what are these pigments?
Well there are several hundred of them, increasingly fast now.
It isn't possible to go through all of these but just by way
of illustration I've just listed
up the seven most common blue pigments used and also
at least 20 others in widespread use which are blue.
But this is the sort of information
which is readily now available.
We know what the chemical name is, the formula.
We know the origin of the blue color in each case.
It's different for different pigments and that's a branch
of inorganic and physical chemistry
that was heavily studied in the 1950's and 60's and 70's to find
out from where the electron was going and to where it was going
in the molecule and it was always going to be in the red,
absorbing wise, if the thing is blue.
So we know all about the origin of the color
and this column simply lists whether something is a mineral
or whether it is synthetic.
And if it's synthetic I've listed the year of manufacture,
fashion blue here, 1704.
Copper [inaudible] blue to 1936 so the message I want to get
over is that if you find Egyptian blue on an Egyptian
from Paris supposedly dating
to 1250 B.C. that's bad news for the proprietors.
Now the technique I'm talking
about is something known as Raman scattering.
Now you may or may not wish to know about this but you're going
to get one slide anyway.
It goes back to Lord Rayleigh who in 1870 was the first person
to realize that if he took a beam of light of one color,
one frequency, and shot it, he was interested
in gases, it would interact.
Most of the light goes straight
through cause it misses the molecules but some
of it strikes the molecules
and the molecules reradiate the same frequency in all direction.
It's a very weak affect and it happens to relate
to the fourth power of the frequency
that you used in the first place.
That means blue scatters better than red.
Now we're all well conscious of that
because it is rays scattering from molecules in the sky
which makes the sky blue, molecules of oxygen and nitrogen
and what have you, they scatter much more effectively
in the blue and they scatter in all directions
to make the sky blue so that
when the sun sets it doesn't all go black instantaneously.
So ray scattering is well known but it's not
in the molecular sense very useful.
We had to wait until 1928 and to C.D. Raman in Calcutta
who made the key discovery
that it wasn't just the original frequency
that a molecule scattered, it's the original frequency plus
or minus some other frequency related to the molecule.
And in fact what I'm going to be talking
about is what's called the Vibrational Raman Effect.
That is you see plus or minus little sidebands.
You've got the Rayleigh line here and then in pairs going
out you see these some difference frequencies
which relate to the vibrations of the molecule,
so highly specific to the molecule they really do act
as fingerprints of what you've hit.
So I can shoot this beam that anyone's top over here,
I won't do it but this man's pretty blue top there
and the signal will come back telling me exactly what pigments
he's got in that.
I mean I haven't got the detecting system
and that's what's tricky but that's the basis of it.
How do you do it?
Well in Raman's day he actually used sunlight,
which is not monochromatic, not single frequency
and as his detector he used his eyes.
Well this was a proof of principle experiment he did.
To do it he quantitative, you've got to have single frequencies
and you've got to have a really good source,
a high power densely source in watts per square meter.
And of course we've gone from the eye to photographic plates
to all sorts of lasers and things
and increased the power density by many orders of magnitude.
And then in terms of detectors for the use
of the eye is extremely limited and we've gone
to many other sorts of detectors finally to something,
which is shown here, is a CCD detector.
That means charged coupled device, something designed
by physicist in order to see stars in distant galaxies
and that will give you an idea of the fantastic sensitivity
that is on the modern Raman detector.
So this is the sort of system that much
of what I will talk about looks like.
The laser beam comes in the back of this instrument,
shoots along here, is filtered
so there's only one frequency comes here.
We link it to a microscope so it's then deflected down there
to the stage and that's where you put the sample.
It then scatters back up here,
comes right through this optical system into this CCD detector
which then tells you what these summon difference frequencies
are and you then go to a library of Raman spectra which we
and other people have developed
so that this can all be looked at and identified.
Well we first looked at anything artistic sort of 15
or more years ago when somebody brought us a little thing
of this sort here.
This is a historiated letter R, which you can see here.
Here's the R, historiated means there's a story being told
in the middle of the letter R
and the letter R is the first letter
of the first word on a page.
And the question was,
I mean this was obviously the Archangel Gabrielle bringing
good news to Mary but the question
that we had was what were these two blues here?
That's what the library wanted an answer to.
And we could tell very quickly
that it was the same pigment azurite,
a copper based carbonate.
And the difference in depth
of color simply related to particle size.
The pale is 3 micrometer sized grains
and the deep blue is 30 micrometer sized grains.
And then we could run through and we looked at these
as malachite, that's basic white lead, basic lead carbonate.
This is vermilion, mercury sulfide
and the yellow is a lead II yellow.
We also looked at the black and that's revealing
in some interesting way because we had assumed
that that would be carbon black, a single material
but in fact it's actually a mixture of different pigments,
each one of which absorbs light in a different frequency
through the visible so you get the net effect of being black
by having this mixture.
I don't know why someone would do that but that's what
in fact they have done in this particular case.
And this also shows the benefit of coupling the Raman system
in with a microscope because you can look at these,
this is obviously a pigment mixture, and you can move
across you see and come onto every pigment grain
and tell what each component is.
And the spatial resolution is about 1 micrometer,
that's 1 millionth of a meter.
So you can go onto that yellow grey [inaudible] which is
about 2 micrometers across, get the signal back from that
and it's not interfered with by all the other pigments
around about.
It's a very good, that what we mean by spatial resolution,
the laser beam will focus under each pigment grain in turn
and you can find out what each component is.
Most techniques won't go anywhere near doing that.
The Raman spectra themselves look something like this
and this is another very early one that we looked at,
a Paris bible coming from the Czech Republic, in fact.
Here we have azurite [inaudible], lapis lazuli,
realgar, lead white, red lead, malachite and vermilion.
That's eight different very common pigments
and as you see each of these is what we would call a Raman
Spectrum, that is a listing, a sort of scanning
of all frequency differences.
And so if you see down here the Rayleigh line is somewhere near
the edge here.
It's been cut off the figure
because the ray is a much more effective scattering business
than Raman is so you don't want
to go take the detector anywhere near the Rayleigh line otherwise
you're liable to kill the detector.
So coming up quite close to zero,
actual zero would be the Rayleigh line except we've cut
it off from there.
These are the peaks, the difference peaks.
I'm not showing the sun but they come out as I said like this
in pairs and these are characteristic
of the compound you're looking at.
So there's eight pigments there.
Everyone of these spectra is different from every other one.
They're easy to get and so they define what the pigment is
uniquely and quickly and they do it with great spatial resolution
and so on and it can be done [inaudible].
Of course you can also look at pigment grains
if someone can bring those to you but sometimes you have
to look [inaudible] and you can do
that under this microscope system.
Well many of the pigments have several books written
about them.
I mean this is a big and an old business,
writing about pigments.
I actually, long before paying any attention
to artwork I was intrigued with lazurite, 40 years ago,
wondering why it was blue.
Well this is the mineral, lapis lazuli,
which comes from a very remote part of Afghanistan,
in the hills and that's the mineral.
The blue part is what's known as lazurite
which is the intriguing part, which is extremely valuable.
It's worth more weight for weight than gold
in the Middle Ages because of the brilliance of the blue color
and the permanence of it and its stability
up to high temperature.
The rest is calcite or something like that
and then there are flecks of iron pyrites in there,
slightly brownish, and there are a few other trace impurities
in it.
The whole history here related
to the very difficult extraction business to get hold
of the mineral lazurite after lapis lazuli.
There was not enough Lazurite in the world for the artists
and that's why it was so highly priced.
And the French government offered a prize in 1824
to anyone who could make it.
People knew it was a sodium aluminosilicate,
which is only one step removed from [inaudible], which is known
and prized because it's white and you make China Clay it is.
So they knew essentially what it was, at least 99%, 99 1/2% of it
and there's a little bit of sulfur there.
Well that's the bit that intrigued me 40 years ago,
sulfur is obviously yellow so it isn't sulfur itself.
And after quite a lot of research, which I won't go into,
we turned out that the chromophore,
that is the thing that's trapped in the cubic cage
of lazurite is S3-, it's a SSS.
It's a bent sulfur species,
trinuclear species carrying one negative charge and it's found
in a few other places, well in crystals and sometimes in melts
when you melt potassium thiocyanate.
So we know what that's caused by and if you go
into the business you can actually make it.
That's the synthetic form which is low
and it's ultramarine blue.
There is also ultramarine green, violet,
pink and an ultramarine selenium,
when you get other radicals trapped in the lattice.
So all of that is now known about.
The ultramarine blue was made in whole for well over a century
and have now sold out to the French actually.
It was a big business when I was a boy I know
because you didn't have proper washing machines then,
you had a copper.
And the clothing that went into the copper always had dolly blue
or reckitt's blue in it
and that's a little muslin bag containing precisely that,
the synthetic form of lazurite and they made tons and tons
and tons of it and it's just [inaudible] and got entailed
in the fabric by a complimentary optical affect.
The yellowing of the whites, that was complimented
by the blue of the ultramarine blue, dolly blue.
They may still put that in some washing powders actually.
I'm not sure now.
Now I don't want to develop onto stories about the pigments
but there are vast volumes on all of the pigments.
Let me turn to some of the things that we've looked
at in recent years so that I know what we've,
so you know something about what we've looked at.
The Lindisfarne gospels, did I just check the time, yes,
okay the Lindisfarne Gospels at the British Library,
this wonderful bit of artwork made by Bishop Eadfrith in honor
of St. Cuthbert on Holy Island, Lindisfarne Island, back in 715.
A fantastic piece of work.
It took him six years to do it
and it's absolutely brilliant artwork.
You can see it yourself if you wander in on the ground floor
of the British Library.
This is Eadfrith's copy that he made.
It was a copy of the Saint Gerome's version of the gospels.
And this is actually the prefatory page.
And I'm no Latin scholar
but with my handheld I can read two words
and it says n-o-v-u-m o-p-u-s, new work.
I can't go beyond that but there are plenty of experts
who will take you well beyond that.
I just want to emphasize that this is a wonderful piece
of artwork, brilliantly colored
and there are many curiosities about it.
It was translated also from the Latin into Old English,
240 years after this, 715,
and there you can see the translation written here that's
known as interlinear gloss, interlinear, between the lines,
gloss means translation.
I think it was controversial when that was added
but it probably isn't now,
probably regarded as interesting.
So this is the Matthew, Mark, Luke and John
and again it's brilliant artwork.
We had a look about it at the British Library
and I'm not going to go through all the pigments that we found
but I was more interested in the one
that we didn't find cause the British Library at the label
up in some detail about lazurite and showing
where it comes from and everything.
That was the only pigment they mentioned on the card
in front of the actual bible.
Actually that's the only pigment that is not there [laughter]
and when we looked at it it was quite clear immediately
that this was indigo, which you extract from woad
and it's a well known and grown and has been for 1,000 years
or so in England, especially on the Eastern side.
What was there was definitely lazurite, I'm sorry,
it was definitely Indigo.
It was not lazurite.
And when you come to think about it it was going
to be incredibly difficult to get a hold of lazurite,
I mean did the monks really know it existed?
How would they have gotten it from the Hindu Kush Mountains
up to Northumbria in 715?
It beggars belief so that's the sort
of strange information found there.
We looked at the Tours Gospel,
825 A.D. Now this is actually the cover.
It's an oak cover.
It's got an embossed silver front to it.
It's got enamel in the corners and it's got gemstones,
12 of them, all around the outside.
The interest there is in what the gemstones were
because it was realized that in past centuries there'd been some
light fingered inspectors and lookers at the gemstones.
We brought in a pocketful of colored glass and sort
of substituted the gemstone with colored glass.
They didn't know what happened here,
well we still don't really know but we looked at those.
There are only four different stones there, there's amethyst,
emerald, iron garnet and sapphire.
They are still there.
Whether they were the original ones we don't know
but at least there's a marker now if anyone wants
to substitute those for colored glass.
We've actually looked at eight Gutenberg bibles,
or pigments from them.
These bibles also on view and very easy to see
at the British Library.
Guttenberg ran these, the first one in Europe,
to use movable and reusable type.
He made about 180, about 48 survived
and there were [inaudible] pigments to illuminate this
with because Gutenberg ran off as it were the black
and white copy you brought back from him.
If you wanted it illuminated you had to find yourself an artist
and then he wanted payment for all the pigments so you had
to decide whether you wanted to put the most expensive of all,
Lazurite, on it or not.
For this particular one, the one we studied in greatest detail,
also you could see in the British Library,
is the George III copy which went in about 1828
and it's superbly illuminated.
It's got images of foliage, flowers,
birds and fruit crawling around the main body of the columns,
which you can see here.
I mean this is absolutely brilliant artwork.
Well we have looked at that and of course these are big volumes,
I mean they're 25 million pound jobs.
You've got to be very careful with them and you've can't sort
of try to bend the back open a big more
to get it under a microscope.
You have to design the right sort of equipment
which will hold it at an optimum angle
and then use a mobile Raman spectrometer, such as this here,
and that's a laser which shoots down onto the page
and then you collect the light back from that
and that tells you what pigments are actually used.
Well this is what we found on that particular one.
We've looked at eight of them from different parts
of the world and there are at least two others in the UK,
one at Easton College and one at Lambeth Palace
and we've seen the pigments from them too.
So we're still working on this trying
to compare whether the palette used was always the same
from one Guttenberg bible to another.
You can also look at paintings,
I mean water paintings quite easy to look at.
Ones that might have any varnish on them are much harder
because the varnish fluoresces and that's completing technique
but one did turn up in London in 1995 and I can't go
into all the history of this because it's big
but Vermeer's had particular problems with them
because in 1947 someone was convicted of forgery in Holland
who apparently made about seven Vermeer's
which the experts couldn't identify
and thought they were Vermeer's so any Vermeer
which didn't have an absolutely perfect history to it,
such as the ones that have been in museums or libraries
since the year or in Buckingham Palace or wherever,
there's no problem with those but one's
where there's a little gap in the knowledge
about where they were held, anything of that sort had a bit
of trouble after 1947 and this was one
that came up in that category.
And Libby Sheldon, UCL History of Art, looked carefully at that
for many years with some of her assistance and we had it
at one stage just to look at too.
And this shows that Vermeer under the microscope
of the Raman microscope
and easily you could see how one could track ground there
and see what pigments are.
There're actually only two which really matter
which are lead-tin yellow type 1 and lazurite,
which are consistent with its being a Vermeer.
This is one of the problems in this kind of work,
if you find something that shouldn't be there,
that rules it out instantly.
If you find things that should be there,
well okay, that's fine.
It doesn't prove anything but you've then got
to do a whole host of other measurements
to see whether you can establish it
as it were beyond reasonable doubt.
Postage stamps are another form of artwork.
[noise]
There was one, we've looked at Mauritius stamps
and these Hawaiian ones.
The Mauritius ones two days ago in the Times,
there was a photograph
of an 1847 Queen Victoria head blue [inaudible] stamp that went
for over a million pounds so you do realize
that postage stamps are worth an awful lot of money
if they're rare and it's worthwhile trying to forge
if you're any good at it.
Well the Hawaiian missionary ones were in that category,
1851, and there was a group known
as the Grenelle Missionary Stamps which were judged
in 1918, 43 of them were sold, were judged to be forgeries.
Fifty five turned up again in 2002 in London
at the Royal Philatelic Society and we linked in with them
to have a look to see what we could see.
Could we see any defining feature which might show
that they are, that's the stamp that was photographed
in the Times two days ago that went for over a million.
This is the Hawaiian Missionary stamp
that we looked at with some care.
It's a .13 stamp shown here.
The blue is Prussian Blue and then many
of the other colors are to do with the Franking, the reds,
vermilion and hematite and there's carbon black there also
and there's nothing wrong with that.
That wouldn't tell you whether it's a forgery or not
but we looked at a cross section here and so the stamp,
this is the edge of the stamp.
It's been sliced through and that's the edge
and on this side is the Prussian Blue.
And this is the fabric of the stamp and what we discovered
over here was small grains
of ultramarine blue in the genuine ones.
And so the stamp maker had put ultramarine blue
in as an optical brightener because paper yellows with age
and so you keep, if you put
in in the first place some complimentary color,
and ultramarine blue is the ideal thing,
that's what actually happens there.
The forged ones, as far as we could see,
never had the ultramarine blue in.
Now you can move across and look
and we have studied a large number of icons of this sort.
This is Saint Athanasius of Mount Athos
in a really bad state of repair.
Mount Athos is on the far Eastern corner of Greece sort
of facing the Dardanelles.
It has 43 monasteries there
and hasn't been a female there for 1,000 years.
I'm not sure what you read into that
but there's not been a cat either.
But I did go there one Christmas and if you really want
to see Monk's driving 4X4's in black capes in hat
up snow roads, that's quite an experience.
But the actual churches left quite wide open
and so anything hanging in these churches has quite a lot
of exposure to the elements and they do degrade badly.
You can see this one has had a terrible time.
It's been over painted and even some of the
over paint has fallen off.
And you can shoot a laser beam down in various places and see
and track and see what the pigments are
at different depths.
You could also take samples, cross sections
and this is a cross section of the Saint Athanasius one.
And if you look then this is the wood.
That's the original painting, that layer.
Then there's a point layer, kind of ground layer.
Then there's the varnish layer
and then there's the over painting.
Now through all that mess you can learn some things.
That red crystal there gives this spectrum,
which is caput mortuum, a form of iron oxide.
The blue gives this spectrum which is azurite,
a copper based carbonate.
Then there's a lot of white material there
which we've identified.
Then there's the varnish and then
up here the three gives you this spectrum which is carbon black
and then up here the four, that's zinc sulfide in the form
of lithopone, which again is a modern pigment.
You expect the modern ones to be on the over painting.
So we can tell them what's there.
Of course it's up to them to decide what they want to do,
whether to restore the surface painting or take it all down
and restore the original painting.
I suspect that many of these icons are in such bad order
that they can't do any restoration at all.
You could also look further at other things,
archeological treasures.
And it so happens that the V&A Museum has a vast,
several hundred items from Samara, which sort of collapsed
around 892 A.D. And all wonder of Samara was buried
for 1,000 years or so until certain experts went out,
German experts went out and brought back
to Europe several thousand items of broken ceramic work.
And the V&A museum got about 300 of these and they've been crated
up for almost a century.
We managed to get the first look of these just two years ago.
And you see these fragments are,
because they've been buried suffered no damage
after they fell apart, they're actually in very good order
and it's no difficulty
in identifying what these pigments are and they turn
out in general to be more or less the same pigments
that are used in artwork.
And there's no end of experimentation
that could be done there at the V&A if they wish to do it
on the other 300 or so things they have.
And they also passed onto us a stucco fragment
from the Alombra, supposedly plaster work of 14 century.
So we had a look at that and especially at the blue
in these grooves here and that is unambiguously passion blue,
first made in 1704 and so the only thing we can tell them is
that either this is a replica or it's been repainted
and so that part is certain.
We can't tell them more than that.
It should have been lazurite,
would originally have been lazurite to go through Grenada.
There's some other things
which were much more obviously forgeries.
I had four Egyptians come and see me once and it turned out,
again this is the sort of dinner table conversation.
It was quite a long story and very funny in parts.
But these four Egyptians at the end of the day wanted
to sell 100 Papyrus at 2 million pounds each in London.
And this is one of them and that's another one.
And I had a quick look at that white worried me
and I had someone, Lucia Bergio [phonetic] who's actually worked
at the V&A, she was very keen to look at papyrus.
UCL wouldn't give us any at that stage.
They had plenty.
So we had a look at this one and it has gypsum
and calcite on it, no problem.
They are minerals.
You'd expect that.
It had anatase on it.
Anatase is a mineral also
but as a mineral it's always deeply colored.
It's either black or deep blue
or can be brown, all sorts of things.
I'd never seen native Anatase white.
We are surrounded by it.
Normally there'd be, even back here would be anatase
because it's #40 chemical in the world, always in paint,
whatever the color because it scatters light extremely well
and they sell it from how white they can make it.
Well that white on Nefertiti's dress was very white.
We looked at it and we know from the pigment, grain size
and everything that that is certainly synthetic.
They were unimpressed with that information
but they were even less impressed of the fact we found
about seven other pigments
which were also modern including the [inaudible] blue, 1936.
It's not 1250 B.C. and that's what the peachy museum ones
looked like.
I mean they're brown because they're mainly iron oxides.
You'd think I'd never look at another Papyrus
but I did have a friend
in the states call me a few years ago and say look.
He'd been to Luxor and bought
from a very posh looking shop quite a nice Papyrus
and he was thinking of leaving it in his will to his son
and he was going to fly it over from the states.
Well he did do that but we didn't even need to get that one
under the Raman microscope,
although that certainly confirmed it.
We got it under an ordinary microscope and that had rows
of yellow dots, rows of red dots and rows of blue dots.
It was clearly the product of an inkjet printer.
[laughter] So I had to advise him he probably didn't need
to mention it in his will.
So I'll just finish with one other example here,
the so called the Spanish forger who was extremely successful.
What was forged was Jorge Ingles,
a 15th century Spanish painter who was greatly admired
and what the chap, they started getting suspicious
because there were too many of them in circulation
and coming up for sale.
And if you look at them you find that they're all
on medieval musical pages, at least one side is musical notes,
staff lines and initials and but definitely medieval,
medieval paper or velum and what have you.
On the other side was scraped clean and he'd do his forgery
on that side, by about 1950 they were realizing
from stylistic grounds there were things wrong
and there were quite a lot of these about.
The V&A museum bought five of these only about a couple
of years ago and we were lucky enough to have a look at them.
Now they're actually very attractive.
The chap made a really good job of them.
And you can see there are the five and this is the Verso
with the medieval inscription still on it.
They're there and there's no problem with this.
But if you actually put these apparent forgeries
under the microscope what you find is this,
there are some undateable pigments
but there are four modern pigments.
There's chrome yellow 89, shields green 70 and 75,
emerald green 1814, ultramarine blue 1828.
These four are signatures of something post 15th century
and so now a days if any more of these crop up you just put it
under a Raman microscope and you will find these four pigments
for sure.
So that's the first scientific investigation of these.
I'll just finish to show you what happens
when something goes wrong in the atmosphere.
This is also at the V&A museum, a pair of dividers here,
German watercolor 1585, what's happened to that hand?
If you look more closely at the hand you'll see
that it's all blackened and the parts that would be brightest,
high lit as it were, are the blackest.
And if you look at those in the black, it's lead sulfide
and it's, what the artist did would make those the whitest
part, basically a carbonate.
But there's been hydrogen sulfide around either
from the atmosphere or from bugs eating
up the sulfur containing binders and generating hydrogen sulfide.
So that's a really bad example of what can happen
in degradation circles and this seems to be about the ultimate
that this particular one,
goodness knows how much hydrogen sulfide got around there
to attack the white lead.
So I think that just gives an idea of this particular way
in which science now impinges on artwork.
[applause]