Karl P. Platt on sat 29 nov 97
Ok, what follows may be literally garbled. I hope not, but I'm always
worried about the Gates Effects. This intends to describe the nature of
the color produced in Cu red glazes/glasses and offer tips, etc on how
to develop them. If you find this the following is not legible please
advise and I'll send a repost from the Unix box at the University --
where we have nothing to do with Bill Gates.
KPP
Cu red glazes are a favorite topic of mine. Really.......
They belong to a highly unique group of colored glazes. The colors given
are highly distinctive and have been highly prized in history -
everyone's heard about the Chinese guy who died taking the "secret" to
the grave with him, leaving the Emperor quite disgruntled. Making Cu red
has to be done according to a number of rules - no secrets here - yet
the origin of the color has been controversial - even up to today.
Cu red glazes are based on adding Cu into the glaze as an oxide and then
exposing it to a reducing firing. If a sample of the glaze is drawn from
the kiln at full heat it will show at most a light straw color, but it
won't be red. The red is produced on cooling by crystals which come out
of solution with the glaze. It is the nature of these crystals that has
been the source of controversy.
Up until 1960 or so it was widely held that the color was due to
metallic copper crystals. Then came Atama Ram and Prasad, who suggested
that the color was actually due to Cu2O (red copper oxide). Ram and
Prasad's paper makes for very interesting reading on a number of levels
- I wish I had a copy of it here! Recognizing the difficulties had in
making Cu Red, and the desirability of the color as used in bangles in
India, they set out to study and refine the parameters of it's
development. In the course of their work they came to the Cu2O
conclusion, but their work was criticized because they added large
amounts of Cu (up to 4.5 wt%) to their glasses.
However, through their work they did obtain delicious and repeatable red
glasses which showed all the characteristics of being colored by Cu2O.
This subject has been batted around further still. Just 10 years ago I
read Tichane's book on Copper Reds in which he concluded red was given
by Cu-metal only. We had a lively exchange of letters shortly thereafter
(no internet for us peons then), in which he vigorously defended his
view - which was based on hydrogen reduction of a glass/glaze.
Through my experiences, I (still) do not believe that Cu-metal is the
only player in Cu Red glasses, but that Cu2O is always present in some
measure and when it predominates, the color is best.
Cu belongs to a group of metals known as the Nobel Metals. That is, they
aren't especially ready to combine with oxygen. The Nobel Metals are Cu,
Ag and Au. In this order they are pregressively less likely to form
oxides. Cu oxides, of course, are well known. Ag oxide is rarely seen
and Au oxide is never encountered in normal circumstances.
Ag or Au are added to a glaze, it is not necessary to employ heavy
reduction and the metals proper form on cooling. In the case of these
metals, the colors produced are strongly dependent on the size and shape
of the crystals formed. Copper (Cu) is a little different. While Ag or
Au, which would rather have little to do with oxygen, requiring little
or no added reduction during firing, Cu which comfortably forms oxides,
indeed, needs to be reduced.
Au (gold ruby) is very rarely used in Studio Ceramics. I can't think of
anyone hand-rolling their own Au reds, but they're are Au ruby
overglazes and glass enamels commercially available. Most of these are
based on soft fritted lead glass and they're not cheap - not because
they have huge amounts of Au in them ( there are very tiny amounts), but
they're tough to make.
Ag can make almost any color in glaze if you know how to manipulate it,
but usually it gives yellow. Ag is essentially never used in modern art
Ceramics, but the ancients used it widely - especially the Persians who
developed fantastic lustres after the collapse of Rome --- the artists
had to go somewhere.
Cu can do a number of things when added to a glaze. In alkaline glazes
it yields a very distinct blue color (Cu2O). In less alkaline glazes
it's green. (CuO). It can also give red colors on cooling when fired
under reducing conditions. However, you can't simply chuck Cu-oxide into
any 'ole glaze and expect it to come out red. The following factors come
into play:
1. Composition
2. The presence of tin oxide
3. Reduction
4. Cooling and sometimes reheating
In terms of composition, the glaze needs to be able to support a couple
of things. First is the solution of Cu. To achieve this it needs to have
things in it that are friendly to the presence of metal. The best of
these is PbO - I can hear the gasps now. Bismuth is another and there
is, of course, tin.
Tin oxide is added to Cu red glazes in amounts way beyond what's
necessary to promote good solution of Cu in the glaze - many
compositions contain up to 4 wt%. This is because tin is volatile at
high temperature and a lot of it leaves the very thin glaze film. This
is one of the reasons that application of the glaze is so important and
why really thin films often fail to develop a nice red - when color
forms at all in a tin depleted glaze it often has the color of liver
instead of a crisp red.
Tin does a couple things in the glaze - particularly in non-lead glazes.
First it improves the solubility of Cu. Metals, per se, aren't really
very soluble in glaze and if you can't get it dissolved, it can't very
well be precipitated in any organized fashion. Second, on cooling, Cu
tends to attract Sn atoms from the glaze. These atoms sort of "coat" the
crystals as they are developed and thus serves to control their size by
limiting the attachment of further Cu atoms to the crystal. Third, to
the extent that Sn has limited solubility in SiO2 or B2O3 based glassy
material, it probably also serves to provide nuclei on which the
coloring crystals can grow.
The amount of Cu necessary to develop a good red depends on how much of
it can be dissolved. Many pottery glazes contain what I feel is a lot of
Cu-oxide in the batch. The best reds always contain the least amount of
Cu.
Reduction is the critical step in producing a nice Cu red.
The thermodynamic considerations of all of this are tedious, and it's
not worth going into all of it here, but the sum results are:
There is some degree of reduction at which Cu2O will be saturated in the
glaze in in an amount greater than Cu-metal. This is the point you want
to achieve to obtain the best color.
If reduction is too strong an abundance of Cu metal is formed and the
color is murky. If it's too little, some of the Cu2O is sacrificed to
CuO, and the color is also less than optimal - in the worst case you
begin to see green in the glaze.
The ideal degree of reduction will be a little different for each base
glaze. I don't have time to crunch the numbers to give a fuel/air mix,
but
necessarily need to be done by eye, nose or sound, but can instead be
metered by a variety of means all of which are cheaper than a kiln full
of muddy looking Cu Red glazes.
Any glaze can only dissolve so much metal. Usually this amount will be a
lot lower than 0.5 wt%. With Cu Reds Less is More is often true.
We've established that there is some degree of reduction at which the
amount of Cu2O is at a maximum and that this is where we will get the
best color. Establishing this condition in the glaze can be done several
ways. Some like to begin reduction early in the firing - around 1700 F
or so and maintain this degree of reduction through the end of the
firing. This is fine. Others like to reduce the pi$$ out of the kiln for
a short time at high temperature. This can work, too, but it never goes
as well as the former approach.
The color is formed by the precipitation of Cu and Cu2O crystals on
cooling. Usually glaze is cooled slowly enough to
f r o m w h a t i s k n o w n a s t h e M i e S c a t t e
r i n g T h e o r y u s i n g t h e o p t i c a l p r o p e r
t i e s o f C u - m e t a l . M i e t h e o r y p r e d i c t
s w h a t w a v e l e n g t h s w i l l b e p r e f e r e n t
i a l l y r e f l e c t e d f r o m t h e m e t a l s u r f a
c e . I t s r e a l c o m p l i c a t e d a n d w e l l
l e a v e i t t o s a y o n l y t h a t t h e r e d R a
w s on f o u n d b y m e a s u r i n g t h e s p e c t r u m
o f t h e r e d i n h i s g l a s s e s , h e g o t r e
s u l t s t h a t w e r e c o n s i s t e n t w i t h C u - m
e t a l .
H
A m a l P a u l , a n a m a z i n g g l a s s s c i e n t i s
t f r o m I n d i a , w a s w e l l a q u a i n t e d w i t
h R a m a n d P r a s a d s C u r e d S t u d y t h
e w o r l d d i d n t g e t s m a l l j u s t 2 0 y e a
r s a g o . P a u l u n d e r t o o k t o s o r t o u t t
h e c o n t r o v e r s y a b o u t j u s t w h a t i t w a
s t h a t m a d e C u g l a s s e s r e d C u 2 O , C
u - m e t a l o r a m i x i f t h e t w o . H e d i d
h i s s t u d i e s i n a g l a s s m a d e o f 3 0 N a
2 O a n d 7 0 B 2 O 3 - - - t h i s d o e s n o t r e p
r e s e n t e i t h e r a u s e f u l g l a s s o r g l a z
e ,
Cu red glazes are a favorite topic of mine c r y s t a l s t h a
t
i n t h e c a l c u l a t i o n s g a v e t h e p r e s e n
c e o f C u A t a m a r
; p r o g r e s s i v e l y
s h a p e d a t o m s t h a t c a n b e h e l d i n s o l u
t i o n w i t h t h e g l a z e B i g c r y s t a l s c a u s
e l i g h t i n c i d e n t o n t h e g l a s s / g l a z e
t o b e s c a t t e r e d s t r o n g l y , l e n d i n g c a
u s e t o a m i l k y a p p e a r a n c e
* * * * * * * * *
P A U S E F O R T H E C A U S E
M a n , y o u s h o u l d s e e t h i s s u n s e t o u t
h e r e . . . . . . . . . . O r a n g e a n d P u r p l e w i s p
s a n d p u f f s r e f l e c t i n g p i n k o n t h e s
u r f a c e o f t h e M a r A t l a n t i c o w h i c h s h
o w s g r e e n b l u e c l a i r i t y b e n e a t h t h e w
i n d b l o w n c r e s t s o f t h e w a v e s . N o w t h
i s i s s c a t t e r e d l i g h t a t i t s a b s o l u t
e b e s t .
* * * * * * * * * * *
( t h e r e A g h a s i t s d i s t i n c t i v e r u b y .
s h e e t g l a s s ( r e a l l y b e a u t i f u l )
g l a s s t h a t i s m u l t i c o l o r e d i n r e f l e c
t e d l i g h t , b u t y e l l o w / a m b e r i n t r a n s
m i t t e d l i g h t t h a t
E s s e n t i a l l y s p h e r i c a l c r y s t a l s o f A
g m e n w h e n i t c o o l s c o m p o s i t i o n R e a l l y
, t h i s w a s a c c e p t e d a s b e i n g q u i t e o
b v i o u s . a r r e d A t a m a r a m g l a s s I n d i a n w
o m e n e s p e c i a l l y l i k e C u r e d 5 e d g l a s s e
s
B e h i n d A t a m a r a m a n d P r a s a d c a m e R a w s
o n w h o s h o w e d t h a t t h e c o l o r p r o d u c e
d w a s c o n s i s t e n t w i t h t h e r e s u l t s e x
p e c t e d f r o m w h a t i s k n o w n a s t h e M i
e S c a t t e r i n g T h e o r y u s i n g t h e o p t i c a
l p r o p e r t i e s o f C u - m e t a l . M i e t h e o r
y p r e d i c t s w h a t w a v e l e n g t h s w i l l b e
p r e f e r e n t i a l l y r e f l e c t e d f r o m t h e m e
t a l s u r f a c e . I t s r e a l c o m p l i c a t e d a
n d w e l l l e a v e i t t o s a y o n l y t h a t t
h e r e d R a w s o
b u t i t i s e a s y t o m e l t . N o t i n w a s a
d d e d t o t h e g l a s s a n d t h e a m o u n t o f
C u t h i s g l a s s h o l d s a r e v e r y l o w u p
t o 0 . 1 3 w t % C u t a k e n a s m e t a l .
M y e x p e r i e n c e s h a v e b e e n c o n s i s t e n t
w i t h h i s r e s u l t s .
I t w o u l d b e u s e f u l t o a s s e m b l e a n d e
x a m i n e s t u d i o r e s u l t s , t o o . T h e r e s
a g r e a t d e a l o f e x p e r i m e n t a t i o n i n t
h e a r c h i v e s o f t h e l a s t 2 5 y e a r s w e
c o u l d p r o b a b l y l e a r n s o m e t h i n g f r o m .
t h e n o b e l s d o n t m i x w i t h t h e p e n
n y - s t i n k a r d s o Y o u c a n b u y A g 2 O f r o m t
h e c h e m i c a l h o u s e , b u t a p a r t f r o m t h
a t i t s n o t s e e n m u c h t h e f i l m t h a
t d e v e l o p s o n y o u r A g t a b l e w a r e i s n
o t p r e d o m i n a n t l y o x i d e . u n t e r e d i n n
o r m a l c i r c u m s t a n c e s , t h i s i s t h e s o u
r c e o f v a l u e i n g o l d .
R e c a l l i n g o u r d i s c u s s i o n o n R e d o x , w
e c a n s a y t h a t t h e o u t e r e l e c t r o n s o
n t h e s e m e t a l s a r e p r e g r e s s i v e l y m o r
e r i g i d l y h e l d m o v i n g f r o m c o p p e r t
o g o l d . G o l d s t a y s r e d u c e d , s i l v e r r
e s i s t s o x i d a t i o n , a n d c o p p e r w i l l g
o a l o n g a n d d i t c h a n e l e c t r o n o r t w
o d e p e n d i n g o n t h e c r o w d i t s i n . W h e
n A g l i k e s a t e e n y b i t a n d A u n e e d s
n o n e . p e r ( C u ) i s a l i t t l e d i f f e r e n t
- - T h e r e s n o t h i n g t o p r e c l u d e a n y o
n e f r o m m a k i n g A u r u b y g l a z e s e x c e p
t t h a t e r r o r s a r e a l i t t l e p r i c e y i
n t e r m s o f t i m e . I n t e r m s o f c o s t , i
t s r e a l l y n o t s o t e r r i b l e a s t h e a m
o u n t o f g o l d n e e d e d i s v e r y s m a l l . G l
a s s m a k e r s u s e A g q u i t e o f t e n t o d e v e
l o p a n u m b e r o f e f f e c t s . T h e s e r a n g
e f r o m y e l l o w g l a s s t o b r o w n g l a s s t
o m u l t i c o l o r e d g l a s s ( i n r e f l e c t e d l
i g h t o n l y , i t s y e l l o w i n t r a n s m i t t e
d l i g h t ) t o g l a s s w h i c h d e v e l o p s a m
e t a l l i c s h e e n w i t h r e d u c t i o n . T h e y e
l l o w a n d b r o w n g l a s s a r e c a u s e d b y e
s s e n t i a l l y s p h e r i c a l c r y s t a l s o f A g
m e t a l . W h e n t h e g l a s s t r u n s b r o w n i
t i s a l s o f r e q u e n t l y t u r b i d ( m i l k y )
.. T h i s i s o w i n g t o h a v i n g f o r m e d l a r
g e a n d n u m e r o u s c r y s t a l s m u s h y a b s
o r p t i o n c h a r a c t e r i s t i c s t e n d i n g t o l
o n g e r ( m o r e r e d ) w a v e l e n g t h s . M u l t i c
o l o r e d e f f e c t s a r e d u e t o t h e d e v e l o
p m e n t o f n o n - s p h e r i c a l c r y s t a l s . T h
e m e c h a n i c s o f a l l o f t h i s a r e n o t ,
h o w e v e r , o u r c o n c e r n h e r e , b u t P e e t
m e n t i o n e d t h e s e a n d I t h o u g h t i t w o u
l d b e w o r t h a b r i e f m e n t i o n a s t h e s
e e f f e c t s c o u l d a l s o b e d e v e l o p e d i
n g l a z e s a s w e l l . T h i s i n d i c a t e s t h e
i m p o r t a n c e o f c o m p o s i t i o n . C u s o m e O
f c o u r s e , i f t h e r e s t o o m u c h t i n i
t d o e s n t a l l d i s s o l v e . T h e g l a s s y p
a r t o f t h e g l a z e w i l l b e c o m e s a t u r a t
e d i n t i n a n d n o m o r e w i l l d i s s o l v
e c a u s i n g o p a c i t y . T h i s m a y o r m a
y n o t b e d e s i r a b l e . T b y e v a p o r a t i o n
C o m p e n s a t i n g t h i s e v a p o r a t i o n i s i m p
o r t a n t t o h o w m u c h t i n w i l l r e m a i n d
i s s o l v e d i n t h e g l a z e . I f y o u u s e t o
o l i t t l e , , s a r e c r u m m y s o l v e n t s f o
r S n O 2 . O n a s i m i l a r f r o n t , n o t i n g t
h a t n o n - B 2 O 3 c o n t a i n i n g g l a z e s a r e t
y p i c a l l y f i r e d t o h i g h e r t e m p e r a t u r e
s w e f i n d t h a t t h e a m o u n t o f r e d u c t i
o n r e q u i r e d i s l e s s i n t h e s e h i g h e r -
f i r e d g l a z e s o w i n g t o t h e n a t u r a l t e
n d e n c y t o w a r d r e d u c t i o n c o n s i d e r e d i
n t h e e a r l i e r R e d o x p o s t . I f t h e c r y s
t a l s g e t b i , t h e g l a z e t u r n s l i v e r
y l o o k i n g . t i n , b u t t h a t s j u s t a
n o p i n i o n A n y g l a z e w i l l d i s s o l v e s o m
e C u
T h i s g l a s s w a s m e l t e d i n a l i t t l e e l
e c t r i c f u r n a c e w i t h a s t r i c t l y c o n t r
o l l e d r e d u c i n g a t m o s p h e r e C O - C O 2 m
i x t u r e s m e t e r e d i n t o t h e f u r n a c e w i t
h p r e c i s i o n g e a r . H e f o u n d t h a t a s t
h e a m o u n t o f r e d u c t i o n i n c r e a s e d t
h e r e i s a l e v e l b e l o w w h i c h n o r e d f
o r m s ; a r a n g e o f r e d u c t i o n w i t h i n w h
i c h g o o d r e d s d e v e l o p e d a n d a p o i n t
( r a n g e ) a b o v e w h i c h t h e c o l o r i s f u n
k y .
A t l i g h t r e d u c t i o n s o m e o f t h e c o p p e
r f o r m e d C u O
H i s c o n c l u s i o n s , w h i c h m y e x p e r i e n c
e c o n f i r m s a r e :
T h e r e i s s o m e d e g r e e o f r e d u c t i o n a
t w h i c h C u 2 O s o l u b i l i t y i s a t a m a x i
m u m
T h e s e i s s o m e d e g r e e o f r e d u c t i o n b e
l o w w h i c h C u O f o r m s . T h i s i s n o t w h e
r e y o u w a n t t o b e b e c a u s e a t t h i s l e
v e l t h e g l a z e w i l l b e u n s a t u r a t e d i
n C u 2 O a n d C u - m e t a l . A s a r e s u l t y o
u w o n t s e e a n y r e d . H o
w
a s i t s s o l u b i l i t y w i l l h a v e b e e n e x c
e e d e d ,
I t s h o u l d b e m e n t i o n e d t h a t b y u s i n
g t h e t e r m C u 2 O i t i s n o t m e a n t t h a
t m o l e c u l e s o f C u 2 O a r e f l o a t i n g a b o
u t i n t h e g l a z e . O n t h e c o n t r a r y , i
t m e a n s t h a t C u + 1 i o n s a r e i n t h e g l
a z e a n d t h a t o n c o o l i n g C u 2 O m o l e c u l
e s ( t h e c r y s t a l s w h i c h g i v e c o l o r ) a
r e f o r m e d .
I n r e d u c t i o n y o u c a n p r o d u c e t h r e e f
o r m s o f C u i n t h e g l a z e . T h e s e a r e :
C u O , C u 2 O a n d C u - m e t a l .
f e e l f r e e . Y A l s o , E l l i n g h a m d i a g r a m
s s a y n o t h i n g a b o u t t h e a c t i v i t y o f t
h e m e t a l / o x i d e i n g l a z e a n d t h e s e e f
f e c t s c a n b e p r o f o u n d . A p a r t f r o m a p
p l y i n g h e a v y s c i e n c e t o m a k i n g C u r e
d g l a z e s w e a l l k n o w t h a t a
T h i s t a k e s u s t o t h e i m p o r t a n c e o f h
a v i n g a s t a b l e c o m b u s t i o n s y s t e m w i t
h m e a n s f o r m e t e r i n g t h e a m o u n t o f f
u e l a n d a i r e n t e r i n g t h e k i l n . P r e c i
s e l y s t a b l e c o m b u s t i o n i s e l u s i v e i
n n a t u r a l d r a f t k i l n s s u b j e c t t o w i n
d , v a r i a b l e a t m o s p h e r i c p r e s s u r e a n
d s o o n . H i g h p r e s s u r e g a s v e n t u r
i b u r n e r s w i l l f u r n i s h b e t t e r r e p e
a t a b i l i t y i n a l l c i r c u m s t a n c e s . A l s o
, o x y g e n a n a l y z e r s c a n n o t m e t e r r e d u
c t i o n a s t h e r e n f o u n d b y m e a s u r i n g t
h e s p e c t r u m o f t h e r e d i n h i s g l a s s e
s , h e g o t r e s u l t s t h a t w e r e c o n s i s t e
n t w i t h C u - m e t a l .
H
A m a l P a u l , a n a m a z i n g g l a s s s c i e n t i s
t f r o m I n d i a , w a s w e l l a q u a i n t e d w i t
h R a m a n d P r a s a d s C u r e d S t u d y t h e
w o r l d d i d n t g e t s m a l l j u s t 2 0 y
T h e g l a z e f i l m i s v e r y t h i n l e s s t h
a n a m i l l i m e t e r . W h i l e g l a z e i s u s u a
l l y p r e t t y v i s c o u s s t u f f , i t d o e s n
t t a k e t e r r i b l y l o n g a t h i g h t e m p e r a
t u r e s t o e s t a b l i s h t h e d e s i r a b l e e q u
i l i b r i u m b e t w e e n C u a n d C u 2 O . I d s u
g g e s t t h a t f i r i n g i n n e u t r a l c o n d i t i
o n s u n t i l t h e l a s t c o u p l e o r t h r e e h
o u r s o f t h e f i r i n g , a n d t h e n a d d i n g
t h e n e c e s s a r y r e d u c t i o n w i l l b e a m o
r e e c o n o m i c a l a p p r o a c h t o o b t a i n i n g
t h e b e s t r e s u l t s . A s w e l l , r e a l l y p r
o l o n g e d r e d u c t i o n h a s o t h e r a f f e c t s
w h i c h m a y n o t b e d e s i r a b l e a t a l l
l i k e d e p l e t i n g t h e s u r f a c e o f N a , a n
d s o o n .
l a z e i s c o o l e d s l o w l y e n o u g h s o t h a
t t h e s e c r y s t a l s h a v e p l e n t y o f t i m
e t o f o r m i n t h e n a t u r a l c o o l i n g o f
t h e k i l n . T h e r e m a y b e t h e w e i r d i n s
t a n c e w h e r e c o o l i n g w a s t o o f a s t a n
d n o c o l o r a p p e a r s . I n s u c h e a r s a g o
.. P a u l u n d e r t o o k t o s o r t o u t t h e c o n
t r o v e r s y a b o u t j u s t w h a t i t w a s t h a
t m a d e C u g l a s s e s r e d C u 2 O , C u - m e t
a l o r a m i x i f t h e t w o . H e d i d h i s s
t u d i e s i n a g l a s s m a d e o f 3 0 N a 2 O a n
d 7 0 B 2 O 3 - - - t h i s d o e s n o t r e p r e s e n
t e i t h e r a u s e f u l g l a s s o r g l a z e , b u
t i t i s e a s y t o m e l t . N o t i n w a s a d d
e d t o t h e g l a s s a n d t h e a m o u n t o f C
u t h i s g l a s s h o l d s a r e v e r y l o w u p t
o 0 . 1 3 w t % C u t a k e n a s m e t a l .
M y e x p e r i e n c e s h a v e b e e n c o n s i s t e n t
w i t h h i s r e s u l t s .
I t w o u l d b e u s e f u l t o a s s e m b l e a n d e
x a m i n e s t u d i o r e s u l t s , t o How much tin to add to
the batch, again, has more to do with the firing conditions than
anything. A kiln in which the velocity of the gases is low will require
less than a kiln with high velocity gases circulating.
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