Jeff Lawrence on wed 21 oct 98
=3Cexcerpt=3EHello to anyone tracking this thread,
On a personal note, submitting one's glazes to a PhD with time on his
hands and a good microscope feels a bit like Monica in front of the
independent counsel. Maybe your glazes look good at 400X, but mine are
embarrassing. Bubbles, patches of unmelted stuff, unmixed gobs, etc. But
knowing the problem is the key to its solution.
I attach Ray's first =22official=22 report below. There were three glazes on
his test vase --
a clear base on the inside (manifested reddish purple spots with green
centers=3B contained .5=25 copper alloy particles)
a clear base on the outside bottom (deep maroon color=3B contained .5=25
copper carb and 1.5=25 tin oxide) and
a high-calcium matt on the upper part of the outside (purple spots with
green centers=3B contained .5=25 copper alloy particles).
COPPER RED GLAZES
16 October 1998
I received the =22sacrificial=22 glazed test cup, and I have been making =
observations on it. The cup was light lavender on the upper half and
dark red on the lower half. It was marked =22clear with shavings=22 on the
inside and =22high calcium matt with shavings=22 on the outside. I will
report to you in the form of a continuing narrative of a =22work in
progress.=22 This is =22part 1.=22
A shard was fractured from the rim of the cup, and it was mounted to view
a cross section of the wall that had been 1.25 inches from the top rim.
The inner glaze (approximately 0.84 mm thick) does not appear to be of
the same composition as the outer glaze. It shows a well-defined
interface with the paste. It has zones of transparent =22rose=22 color, it
shows conchoidal fracture, there is no sign of multiple solid phases, the
color appears to be in solution, it is rather full of bubbles (some quite
large, few very small), there are nearly no bubbles at the interface
(bubbles not formed as a result of glaze/paste interaction), and there is
no dichroism (the glaze is isotropic --- it is completely vitreous). The
sharp edges of the fracture are perfectly smooth=3B ie, there are no hidden
particles imbedded in the glass.
The outer glaze has fused with the paste, giving an indistinct interface.
The glaze shows dichroism and many suspended fine crystalline particles.
There are lots of small bubbles in the glaze. Although the interface is
indistinct, the most vitreous part of the glaze is about 0.48 mm thick.
The glaze gets thicker toward the bottom of the cup.
Upper zones on the outside show a light lavender color=3B however, there is
no color in the outer 0.14 mm of the glaze layer. That part does,
however, still show crystalline particulates. The sharp fractured edge
shows a jagged profile as a result of the crystals. The crystals are
colorless and are 1 =B5m or less in diameter. The color appears to be
completely in solution in the vitreous phase. The lower part of the cup
has a deep-red glaze on it, there are very few suspended crystals, and
there is a much thinner clear layer at the surface.
Observations by transmitted light
Fragments in the form of thin flakes were observed under 1.515
refractive-index (n) immersion oil. This is the index of normal lime
Both glazes show an index of refraction very close to 1.515.
A fragment of the deep red glaze (from the outside bottom of the cup)
shows n very slightly =3E1.515. The red color appears to be extremely well
dispersed in the vitreous phase, and the glaze is much more completely
vitrified than the glaze in the upper half. It does not show any red
particles above the limit of resolution of the microscope, making it
appear to be a solution. However, light scattering increases in clear
red zones when viewed by conical illumination: it shows a Tyndall effect,
proving there is a colloidal suspension. I can not yet tell what type of
colloidal particles are responsible for the light scattering. Red and
green illumination did not show any unexpected differences.
The colorless zone on the outside of the colored glaze is very thin in
the deep-red zone, and there is a very thin (=3C=3C0.04 mm) zone of darker
red between the vitreous colored and vitreous clear zones. There are
many dark green/gray zones inside the continuous red vitreous phase.
Much of the =22green=22 appears to be in solution=3B however, some =
structure is visible. When a mixed green/red fragment is crushed on the
slide, the green-grainy part tends to shatter into many birefringent
crystals that are 2 - 5 =B5m in diameter=3B however, there is some vitreous
green phase. There are absolutely no suspended birefringent crystals in
the vitreous red phase. The red phase is completely vitreous.
The depth of field of a 40X objective is very short=3B therefore, it is
possible to focus up and down through a transparent sample, seeing
components inside the glass in sharp focus.
Optical sectioning at high power showed clusters of black particles and
very, very small light-scattering particles (just below the limit of
resolution) in the colored zones. There are no similar particles in the
birefringent crystals. These clusters and very small to colloidal
particles could be carbon or elemental copper=3B however, their appearance
favors carbon. A few black clusters can be seen inside bubbles in a
fractured surface. A little simple chemical work could prove the
composition of the opaque particles. I'll try to get the equipment and
I currently doubt that the red color is due to colloidally dispersed Cu2O
or Cu0=3B however, some of the colloidal particles that cause light
scattering could be precipitated Cu2O. This would explain why very low
amounts of copper give better results. When you saturate the glaze with
Cu2O and/or CuO at high temperature, solid Cu2O will precipitate as the
glaze cools. Some of the crystals/colloids may be insoluble or
high-melting components of the glaze=3B some may be precipitated copper
oxides. Any precipitate or suspended particles will cause light
scattering. A perfect, clear red glaze would have to be free of small
crystals, bubbles, and colloids.
I believe that the clear red color is most likely Cu2O in solution. Tell
me why I'm wrong.
=B7 An excellent red color is obtained by your approach.
=B7 The red is Cu2O in solution in the vitreous phase of the glaze.
=B7 The glaze is not uniform, containing many zones of green/gray that dull
the effect. Some green/gray zones are large enough to see through the
=B7 The high-calcium glaze is not completely vitrified, producing a =
appearance. This must degrade the brilliance of the system.
=B7 Small crystals, bubbles, and colloidal carbon scatter light, producing
a less-than-brilliant effect.
=B7 There is no colorless zone between the paste and the glaze.
=B7 There may be too many bubbles for good optical effects. Bubbles
scatter light. Is there carbonate in the glaze?
=B7 Make sure the glaze is completely vitrified.
=B7 Protect the surfaces from luminous flame (colloidal carbon).
=B7 Ensure intimate mixing of Cu with the glaze (eliminate lumps that
apparently cause the green/gray inclusions). It may be enough to use
very finely ground Cu2O (Cuprite), but a colloidal material might be
=B7 If you start with Cu+1, less reduction will be required.
=B7 Stay within the solubility limits of Cu2O in the glaze.
=B7 Eliminate any carbonate or hydrated materials to reduce the number of
=B7 Try a little borate to increase solubilities and reduce the time
required for reduction. I have no idea what bad effects this would
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