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copper reds - ray and his microscope

updated fri 23 oct 98


David Hendley on thu 22 oct 98

Hey Jeff, I like reading your reports from your friend Ray and hope you
will continue to forward them to us, but I'd like to make a comment.
You say:
=3E=3EMaybe your glazes look good at 400X, but mine are
=3E=3Eembarrassing. Bubbles, patches of unmelted stuff, unmixed gobs, etc. =
=3E=3Eknowing the problem is the key to its solution.

I say:
I LIKE bubbles, crystals, and patches of unmelted stuff=21
I see thousands of bubbles in my glazes with just a 20X hand magnifier.
At 20X, my copper red glaze is full of bubbles, full of blue-ish areas, and
is speckled with dark red and brown spots.
It's a beautiful glaze.
A completely uniform bright red glaze is not what I want, and, in my
opinion, would be pretty boring.
We're not making sanitary ware here, where uniformity and technical
properties are the only concerns.

David Hendley
Maydelle, Texas

At 09:20 AM 10/21/98 EDT, you wrote:
=3E----------------------------Original message----------------------------
=3E ------------------
=3E=3EHello to anyone tracking this thread,
=3E=3EOn a personal note, submitting one's glazes to a PhD with time on his
=3E=3Ehands and a good microscope feels a bit like Monica in front of the
=3E=3Eindependent counsel. Maybe your glazes look good at 400X, but mine are
=3E=3Eembarrassing. Bubbles, patches of unmelted stuff, unmixed gobs, etc. =
=3E=3Eknowing the problem is the key to its solution.
=3E=3EI attach Ray's first =22official=22 report below. There were three =
glazes on
=3E=3Ehis test vase --
=3E=3Ea clear base on the inside (manifested reddish purple spots with green
=3E=3Ecenters=3B contained .5=25 copper alloy particles)
=3E=3Ea clear base on the outside bottom (deep maroon color=3B contained =
=3E=3Ecopper carb and 1.5=25 tin oxide) and
=3E=3Ea high-calcium matt on the upper part of the outside (purple spots =
=3E=3Egreen centers=3B contained .5=25 copper alloy particles).
=3E=3E 16
=3E=3EOctober 1998
=3E=3EI received the =22sacrificial=22 glazed test cup, and I have been =
making some
=3E=3Eobservations on it. The cup was light lavender on the upper half and
=3E=3Edark red on the lower half. It was marked =22clear with shavings=22 =
on the
=3E=3Einside and =22high calcium matt with shavings=22 on the outside. I =
=3E=3Ereport to you in the form of a continuing narrative of a =22work in
=3E=3Eprogress.=22 This is =22part 1.=22
=3E=3EFracture surface
=3E=3EA shard was fractured from the rim of the cup, and it was mounted to =
=3E=3Ea cross section of the wall that had been 1.25 inches from the top =
=3E=3EThe inner glaze (approximately 0.84 mm thick) does not appear to be of
=3E=3Ethe same composition as the outer glaze. It shows a well-defined
=3E=3Einterface with the paste. It has zones of transparent =22rose=22 =
color, it
=3E=3Eshows conchoidal fracture, there is no sign of multiple solid phases, =
=3E=3Ecolor appears to be in solution, it is rather full of bubbles (some =
=3E=3Elarge, few very small), there are nearly no bubbles at the interface
=3E=3E(bubbles not formed as a result of glaze/paste interaction), and there=
=3E=3Eno dichroism (the glaze is isotropic --- it is completely vitreous). =
=3E=3Esharp edges of the fracture are perfectly smooth=3B ie, there are no =
=3E=3Eparticles imbedded in the glass.
=3E=3EThe outer glaze has fused with the paste, giving an indistinct =
=3E=3E The glaze shows dichroism and many suspended fine crystalline =
=3E=3E There are lots of small bubbles in the glaze. Although the interface=
=3E=3Eindistinct, the most vitreous part of the glaze is about 0.48 mm =
=3E=3EThe glaze gets thicker toward the bottom of the cup.
=3E=3EUpper zones on the outside show a light lavender color=3B however, =
there is
=3E=3Eno color in the outer 0.14 mm of the glaze layer. That part does,
=3E=3Ehowever, still show crystalline particulates. The sharp fractured =
=3E=3Eshows a jagged profile as a result of the crystals. The crystals are
=3E=3Ecolorless and are 1 =B5m or less in diameter. The color appears to be
=3E=3Ecompletely in solution in the vitreous phase. The lower part of the =
=3E=3Ehas a deep-red glaze on it, there are very few suspended crystals, and
=3E=3Ethere is a much thinner clear layer at the surface.
=3E=3EObservations by transmitted light
=3E=3EFragments in the form of thin flakes were observed under 1.515
=3E=3Erefractive-index (n) immersion oil. This is the index of normal lime =
=3E=3EBoth glazes show an index of refraction very close to 1.515.
=3E=3EA fragment of the deep red glaze (from the outside bottom of the cup)
=3E=3Eshows n very slightly =3E1.515. The red color appears to be extremely=
=3E=3Edispersed in the vitreous phase, and the glaze is much more completely
=3E=3Evitrified than the glaze in the upper half. It does not show any red
=3E=3Eparticles above the limit of resolution of the microscope, making it
=3E=3Eappear to be a solution. However, light scattering increases in clear
=3E=3Ered zones when viewed by conical illumination: it shows a Tyndall =
=3E=3Eproving there is a colloidal suspension. I can not yet tell what type =
=3E=3Ecolloidal particles are responsible for the light scattering. Red and
=3E=3Egreen illumination did not show any unexpected differences.
=3E=3EThe colorless zone on the outside of the colored glaze is very thin in
=3E=3Ethe deep-red zone, and there is a very thin (=3C0.04 mm) zone of =
darker red
=3E=3Ebetween the vitreous colored and vitreous clear zones. There are many
=3E=3Edark green/gray zones inside the continuous red vitreous phase. Much =
=3E=3Ethe =22green=22 appears to be in solution=3B however, some fine-grain =
=3E=3Eis visible. When a mixed green/red fragment is crushed on the slide, =
=3E=3Egreen-grainy part tends to shatter into many birefringent crystals =
=3E=3Eare 2 - 5 =B5m in diameter=3B however, there is some vitreous green =
=3E=3EThere are absolutely no suspended birefringent crystals in the =
=3E=3Ered phase. The red phase is completely vitreous.
=3E=3EOptical sectioning
=3E=3EThe depth of field of a 40X objective is very short=3B therefore, it =
=3E=3Epossible to focus up and down through a transparent sample, seeing
=3E=3Ecomponents inside the glass in sharp focus.
=3E=3EOptical sectioning at high power showed clusters of black particles =
=3E=3Every, very small light-scattering particles (just below the limit of
=3E=3Eresolution) in the colored zones. There are no similar particles in =
=3E=3Ebirefringent crystals. These clusters and very small to colloidal
=3E=3Eparticles could be carbon or elemental copper=3B however, their =
=3E=3Efavors carbon. A few black clusters can be seen inside bubbles in a
=3E=3Efractured surface. A little simple chemical work could prove the
=3E=3Ecomposition of the opaque particles. I'll try to get the equipment =
=3E=3Echemicals together.
=3E=3EI currently doubt that the red color is due to colloidally dispersed =
=3E=3Eor Cu0=3B however, some of the colloidal particles that cause light
=3E=3Escattering could be precipitated Cu2O. This would explain why very =
=3E=3Eamounts of copper give better results. When you saturate the glaze =
=3E=3ECu2O and/or CuO at high temperature, solid Cu2O will precipitate as =
=3E=3Eglaze cools. Some of the crystals/colloids may be insoluble or
=3E=3Ehigh-melting components of the glaze=3B some may be precipitated =
=3E=3Eoxides. Any precipitate or suspended particles will cause light
=3E=3Escattering. A perfect, clear red glaze would have to be free of small
=3E=3Ecrystals, bubbles, and colloids.
=3E=3EI believe that the clear red color is most likely Cu2O in solution. =
=3E=3Eme why I'm wrong.
=3E=3ETentative conclusions
=3E=3E=B7 An excellent red color is obtained by your approach.
=3E=3E=B7 The red is Cu2O in solution in the vitreous phase of the glaze.
=3E=3E=B7 The glaze is not uniform, containing many zones of green/gray that=
=3E=3Ethe effect. Some green/gray zones are large enough to see through the
=3E=3E=B7 The high-calcium glaze is not completely vitrified, producing a =
=3E=3Eappearance. This must degrade the brilliance of the system.
=3E=3E=B7 Small crystals, bubbles, and colloidal carbon scatter light, =
=3E=3Ea less-than-brilliant effect.
=3E=3E=B7 There is no colorless zone between the paste and the glaze.
=3E=3E=B7 There may be too many bubbles for good optical effects. Bubbles
=3E=3Escatter light. Is there carbonate in the glaze?
=3E=3E=B7 Make sure the glaze is completely vitrified.
=3E=3E=B7 Protect the surfaces from luminous flame (colloidal carbon).
=3E=3E=B7 Ensure intimate mixing of Cu with the glaze (eliminate lumps that
=3E=3Eapparently cause the green/gray inclusions). It may be enough to use
=3E=3Every finely ground Cu2O (Cuprite), but a colloidal material might be =
=3E=3E=B7 If you start with Cu+1, less reduction will be required.
=3E=3E=B7 Stay within the solubility limits of Cu2O in the glaze.
=3E=3E=B7 Eliminate any carbonate or hydrated materials to reduce the number=
=3E=3E=B7 Try a little borate to increase solubilities and reduce the time
=3E=3Erequired for reduction. I have no idea what bad effects this would =
=3EJeff Lawrence
=3ESun Dagger Design
=3ERt. 1 Box 394L
=3EEspanola, NM 87532
=3Evox/fax 505-753-5913