John B. Paine III on sun 20 aug 00
To all Clayartists: As a result of encouragement from contacts that started
with my posting to about.com on the subject of rare earth glaze coloration,
I have decided to join your e-mail discussion group, and the American
Ceramic Society. My interest in ceramics is at the hobby level,
particularly with ceramic glaze formulation, using non-traditional
ingredients. With respect to ceramic glaze coloration, this particularly
includes the rare earth elements (lanthanides), although my plans also
include examination of several of the colorless lanthanides to be used in
combination with colorants such as copper that may show color change as a
result of the combination. I will also eventually look into the use of
niobium and tantalum pentoxides in glaze formulation, perhaps in combination
with the rare earths. (This is a combination that often occurs in the
mineral world, particularly in granite pegmatites.)
Rare earths are enormously refractory and non-volatile, and non-toxic,
so are outstanding candidates for use in routine ceramic glaze formulation.
What has held them back from use in the past has been cost. This is no
longer a barrier. Due to Chinese development of their enormous rare earth
resources, rare earths are now priced near their historic lows. The
following rare earth oxides are now available for less (or much less) than
the price of cobalt oxide, traditionally the expensive item in a ceramics
colorist's palette: yttrium, lanthanum, cerium, praseodymium, neodymium,
samarium, gadolinium, and erbium. The first seven of these can be had for
less than 20 dollars per kilogram or about 9 dollars per pound - in
5-kilogram lots. Erbium oxide can be had for less than 40 dollars per kilo.
Having said that, it makes a great difference to the pocketbook as to where
one goes to price rare earths. Many of the usual laboratory price lists
quote prices of 50 to 1000 times these levels. One needs to buy directly
from a rare earth specialty supplier, in order to obtain prices as friendly
as those mentioned above. I recently looked at the rare earth source-list
connected to the American Ceramics Society webpage. The one on the list
from whom I have been buying, and which seems to be giving me the best
prices is PIDC (Pacific Industrial Development Corporation).
My own project for rare earth glaze formulation is slowly unfolding.
The first stage was to acquire all of the colored rare earth oxides in
5-kilogram quantities, and this has now been done. A number of them are
also being acquired as the carbonates, since one of the subprojects involves
the formation of rare earth borates under aqueous conditions, to provide
rare-earth analogs of colemanite, both colored and colorless. [By rare
earth analogs of colemanite, I mean this in a ceramics, rather than a
mineralogical sense.] This part of the project has begun, and it appears
that both oxides and carbonates can be made to react with saturated hot
aqueous boric acid, although in some cases, several days are needed to
complete the reactions. I will have to send products off for analysis,
since a search of the literature suggests that this means of borate
synthesis may not have been widely used before, and therefore I may be
getting new phases, whose stoichiometry need to be determined. The
carbonates were particularly important to acquire in the case of cerium(III)
and praseodymium(III), since the usual oxides are partly or completely
quadrivalent, and thus expected to resist reaction with boric acid. Both
carbonates have been reacted with boric acid, and appear to have given
trivalent borates, of as-yet undetermined stoichiometry. Huge excesses of
boric acid have been used, in excess of six moles of boric acid per
gram-atom of lanthanide. By the way, I have no interest in patenting any of
this, which is why I am discussing it openly. I am putting this in the
public domain so that in theory nobody can patent it, and therefore it will
be available for all to use. (I have patents aplenty in other fields to
satisfy my ego.)
Rare earth oxides mostly have the R2O3 sesquioxide stoichiometry, which
may confuse those used to the unitary formulation of ceramic glazes. This
is because the rare earths are very strong bases, similar to calcium or
magnesium in that respect (or even strontium or barium). My first foray
into glaze formulation (since a small amount of work as a teenager in the
1960's) will proceed on the assumption that equimolar replacement of calcium
or magnesium in published recipes by rare earths will give a viable glaze.
A parallel set of models will replace Ca or Mg with the same equivalent of
lanthanide (or 2/3rds of the equimolar quantity).
For those of you who wish to explore rare earth coloration on your own,
the following facts need to be remembered. Rare Earth colors are relatively
subtle, due to narrow absorption lines, and relatively weak absorption
intensities. Impurities such as iron in the other ingredients, or in the
clay below, need to be controlled or avoided to prevent the rare earth
colors from being overwhelmed. It may be that a white clay engobe painted
on a red clay base as a barrier may suffice to protect the rare earth colors
from contamination. I do not have any current experience to tell me whether
iron displays any volatility in firing to the stoneware range, which is the
range of temperatures that I will be targeting. My reading of the
literature suggests that copper and chromium may be sufficiently volatile to
be worth avoiding in the presence of intended rare earth colorants.
Once the problem of potential contamination is under control, rare earth
colors will be found to be mostly invariant with respect to the formulation
of other colorless ingredients, impervious to oxidation or reduction
conditions, or to the temperature reached during firing. Only cerium and
europium might display redox chemistry in a glaze - more about this in a
future posting. Black praseodymium oxide dissolves in silicate melts to
give yellow-green trivalent praseodymium exclusively - this is one that I
personally used in the 1960's, back when it was 40 dollars per pound for 99%
material.
Due to the relatively subtle colors, loading levels in a glaze might
need to be as much as 5 or 10%.
Colors of the rare earths:
Yttrium, lanthanum, cerium(III), gadolinium, ytterbium and lutetium: all
colorless. (Ytterbium is expensive, and lutetium is the most expensive rare
earth of all, about 1000 per kilo, so can be safely ignored.) Yttrium,
lanthanum and gadolinium will be explored for any influence they may have on
non rare earth colorants such as copper. Cerium(III) and Cerium(IV) have
considerable potential to influence or stabilize other colors, by buffering
the oxidation potential of a melt, thereby keeping other ingredients
oxidized or reduced, as needed.
Praseodymium: yellow-green, definitely more green than yellow. The
"Praseodymium Yellow" familiar to you all since the 1960's contains
quadrivalent praseodymium locked into the zircon lattice, and does not
display the usual color of trivalent praseodymium, which is yellow green.
Neodymium: pink to purplish magenta, particularly in daylight or
incandescent light. Light gray-blue to blue under white fluorescent light.
Samarium: should be a light cream gold or yellow
Europium: colorless, but with a brilliant orange-red fluorescence under
black light (oxidizing conditions), but perhaps a bright blue-white
fluorescence under ultra-strong reducing conditions.
Terbium: colorless but with a brilliant lemon-yellow fluorescence under
black light
Dysprosium: lemon yellow, but not very intense
Holmium: variable. In daylight, tawny gold, in incandescent light greenish,
but under trichromatic light, pink
(Holmium oxide is as pink as erbium oxide when viewed under trichromatic
light!)
Erbium: pink (the oxide is a gorgeous baby pink), especially in daylight.
Perhaps on the peach side in a glaze, I have yet to see the results.
Thulium: pale bluish green, probably a color akin to some of the varieties
of celadon, but without need for reducing conditions. The tragedy for most
of you is that this one is expensive: around 800 dollars per kilogram, since
this it the second rarest of all the rare earths. I will however be
exploring this one personally.
Cerium (IV): ceric oxide tends to be stable in melts and of low solubility,
so is an opacifier. The color when pure is a pale yellow, although white
forms are produced if especially fine particled. The color is also
sensitive to traces of praseodymium, becoming pinkish or reddish as the
praseodymium content increases. I do not know whether any deliberate
cerium-praseodymium binary oxides are marketed as reddish pigments. I do
know that Rhodia has developed cerium and samarium sulfides as pigmental
replacements for cadmium sulfide and cadmium selenide, but this would be for
low-temperature applications, not glazes (unless ways have been found to
encapsulate the sulfides to protect them from atmospheric oxidation during
the firing process.)
Enough for now: I have to go the American Chemical Society convention in
Washington, where I have two presentations to give (on other topics than
this).
John B. Paine III
iandol on tue 22 aug 00
Dear John B. Paine III,
Would you please confirm that there are no Radioactive isotopes in =
naturally occurring Lanthanide elements or their compounds. I have read =
that some of these these substances must be handled with special care =
because they are emitters.
I also understand that Chinese parties have already patented lanthanide =
use in the production of fluorescent glazes.
Can you provide enlightenment on these questions.
Ivor Lewis.
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