eutectics

updated tue 24 nov 09

Tom Buck on wed 8 jul 98

Carrie Jacobson:
David Hewitt, John Baymore posted this week some words on
eutectics. I presume you are unclear about what they are saying.
The word "eutectic" comes from chemists/metallurgists working with
metal alloys many years back. The word was invented to indicate the LOWEST
melting point achieveable when two metals were mixed together (either as
powders or as molten liquids), and resultant alloy tested for its melting
point. Here is what a chemical dictionary says:
"eutectic. The lowest melting point of an alloy or solution of two
or more substances (usually metals) that is comprised of the same
components. Eutectic alloys are relatively few; they are particular alloys
that have definite and minimum melting points compared with other alloys
of the same metals."
People who were working in ceramic studies also found the term of
use so they began applying it to the LOWEST melting point of mixtures that
would form glass (ie, a glaze mix) on a clay pot/vessel/form. Like a metal
alloy, the ingredients would yield such a eutectic mixture when combined
in a specific proportion. But, unlike metals (all pure substances
chemically speaking, ie, one molecular species only), glazes with the same
fired mix of essential oxides, could be derived from differing materials
(not "substances" in chemical terms). As a result, the study of
glass-forming materials is quite more complex than that of rather simple
alloys.
To bring order to such complexities, ceramacists use "phase
diagrams" and "triaxial diagrams" to represent the combinations/activities
involved in various glaze mixtures. And to simplfy things for these
diagrams, all components in the mixture have to be rendered in molecular
form, ie, moles of specific oxides, eg, CaO calcium oxide, K2O or Na2O
potassium oxide or sodium oxide, SiO2 silicon oxide (silica), Al2O3
aluminum oxide (alumina) and other oxides. This is similar to a Seger
(unity) Formula which is itself a rendering of a glaze recipe into
proportions of specific molecular species included in the components of
the recipe.
As others have explained, one can make use of "eutectic" data when
designing a glaze from scratch, or one can use "Limit Tables" which
provide a guide to a Seger Formula (ie, moles of essential oxides) needed
for a successful glaze at a particular firing range.
In the end, however, all such knowledge won't guarantee a test
glaze will succeeed on a pot. But it will help point you in the right
direction to retest and obtain a successful new glaze.

Tom Buck ) tel:
905-389-2339 & snailmail: 373 East 43rd St. Hamilton ON L8T 3E1 Canada
(westend Lake Ontario, province of Ontario, Canada).

Tommy Humphries on fri 7 dec 01

I was discussing some glazing points today with a potter friend of mine
while some non potters listened in. When we were taking about the melting
points of various materials eutectics naturally came up and the non potters
became interested and started asking questions, which we tried to answer as
best we could. There was one that I nor my friend could answer and it is
this...

Is there a "reverse" eutectic? That is one that would raise the melting
point of two materials above either of their natural melting points...any
that are applicable to pottery??

Tommy Humphries

Photos here http://albums.photopoint.com/j/AlbumIndex?u=939179&a=6869600

Ivor and Olive Lewis on fri 9 feb 07

Dear Steve,

You are forgiven about the slip over names. That iandol might get =
changed to oandil. Mail frequently starts "Dear iandol..."

My comments about lack of interest related only to the potential to test =
the statement of Mr Michael Cardew about a particular mixture of =
Whiting, Kaolin and Quartz. I have written to Seth Cardew to find out if =
mention of it is made in his Father's original manuscript. My sample =
melted to give a vitreous residue with a suspicion of round crystals but =
the was at Cone 8 over. I was looking for some feedback from our Cone 6 =
Gang.

Some years ago when my interest in Anorthite was aroused I tried to get =
a sample. I was offered a crush of about 100 tons by a UK company =
("...Not worth opening the quarry for less..." ! ). Michael Banks kindly =
sent me a small sample of Anorthosite from his collection as being the =
nearest I might find in the Southern Hemisphere.

I think conversations we have had are raising awareness. That can only =
be a good thing.

Best regards,

Ivor

Joseph Herbert on tue 4 sep 07

I have an observation, or two, about eutectics.

The definition I have in my mind is something like, "the lowest melting
temperature composition for a two or more component (material) system."

The most familiar example of this lowest melting composition is probably the
tin-lead system (solder) where the composition of about 63% tin and 37% lead
melts at a lower temperature (361 degrees) than any other composition from
pure lead (620 degrees) to pure tin (450 degrees). This is often expressed
in 2D graphic form with a composition range across the bottom of a graph
from 100% Pb to 100% Sn and a temperature scale running vertically. At
either end the melting point of the pure material is indicated by a line at
the melting temperature indicating there is liquid metal in the temperature
area above the line and solid metal in the temperature area below the line.
The line separating the condition of all liquid metal from liquid metal
mixed with solid metal swoops down toward the composition of 63% tin and 37%
lead Finally arriving at the 361 degree temperature. Only at either end of
the graph and at that one point in the middle does all the material melt at
one temperature. Pure lead will melt, change from solid to liquid entirely
at one temperature; so will pure tin (different temperatures). Indeed,
while the phase change in pure metal is taking place, the temperature is
stabilized at the melting temperature while heat is absorbed by the solid to
melt (The heat put in during the phase change (melting) is the latent heat
of fusion and is the same as the latent heat of crystallization released
when the material solidifies). All other tin/lead alloys partially melt at
some temperature, exist as a slushy mixture of liquid and solid crystals
while increasing in temperature until a higher temperature is reached and
all the material is melted. On many phase diagrams of the Pb/Sn system
there is a lower curved line separating the temperature where all metal is
solid from temperature where a little of the metal has melted. A comparable
upper curved line (mentioned above) divides areas of all liquid from areas
of some liquid, some solid. The two sets of curves sort of look like
inclined cat eyes joined at the Eutectic point.

Eutectics in other systems are not as dramatic as this but they do exist,
even is really complex chemical systems. A more complex, but familiar,
example is Granite, the rock. The composition of most granite is close to a
eutectic composition for a system containing Silica, alumina, and a mixture
of potassium and sodium oxides. For many granites those four constituents
comprise about 93 percent of the entire rock. So, if you draw up a
triangular diagram with one of each of these components at each corner, the
eutectic for the system will be around the area of 72 - 74 % SiO2, 14%
Al2O3, and 4 - 7 % alkali oxide. When this melted mixture solidifies it
forms crystals of feldspar and quartz, the major constituents of Granite.
(in this kind of diagram the 100% composition for one of the constituents is
at a corner. There are then lines drawn parallel to the opposite triangle
side representing a decreasing percentage of that material until the
material is absent along the opposite triangle side.)

So the eutectic for the SiO2/Al2O3/alkali oxide is pretty close to the SiO2
point, shifted along the 74% SiO2 line so that the remaining 26 percent of
stuff is divided about 2/3 for Al2O3 and 1/3 for Alkali oxide.

All very interesting and a straight forward statement. Let's think for a
moment about how this information is obtained.

Some graduate student or other researcher loads up a container with the
materials the compose the system of interest. He might mix equal parts by
weight, By gram molecular weight, or some other choice. He then takes his
container and heats it to a specified temperature for that particular run.
He has to leave it in the furnace, at that temperature for a fairly long
time to allow the material to be heated thoroughly and to melt if it is
going to. After the time specified in the experimental protocol he probably
takes the container and cools it as fast as he can safely while leaving the
sample intact. If there was melting, he saws the sample with a diamond saw,
makes a thin slice, and looks through a polarizing microscope or maybe uses
an electron microscope to see what melted and how much. That is ONE POINT -
one temperature, one melt composition. On to the next.

So the eutectics for any of these metal oxide systems whether rocks or
ceramic materials (Same Same) are found by a very tedious process of mixing
materials, holding at a temperature, analyzing what and how much melted, and
then making another batch to do the same thing to. Each has to be heated
for some time (hours?) and then cooled, sampled, and inspected. Time
consuming and tedious.

Materials with a large economic interest, like the alloy phases of iron and
carbon (Oh, yeah - steel), get lots of attention, many people making those
furnace runs, analyzing the material from a particular temperature region.
Other materials of lesser interest get less intensive attention. The phase
diagram of the system Fe - C in the range of 0 to 4% carbon has lots of well
defined phases. Many people spent long times looking at all those
experimental points to know where to draw those lines.

Some phase diagrams look very simple, there are few lines separating phases.
This may represent the reality of a very simple system or it may represent
the limited conclusions possible with the information available - you can't
really tell unless you can get the raw data in a paper and see that the
research was 20 data points or 4000 data points.

So, as pointed out by others, the meaning of ceramic phase diagrams to a
practicing studio potter is, at best, tangential. One of the reasons is
easily seen by the example of Fe-C. Additions of extremely small amounts
(fractions of a percent) of carbon to iron cause vast changes in bulk
properties. Addition of really small amounts of a material to a glaze can
have a range of effects, some tragic, some helpful. In the cases of these
small additions which we face continuously because of impurities in the
natural mined materials we use for glaze components, their effects will
never be accurately determined or shown in a phase diagram. Potters end up
having a seat-of-the-pants feeling about some of the possible minor
constituents, may have a observed good result without understanding the
mechanism, may be mouse trapped by a supplier when mining moves to a
different side of the quarry and the contaminants change.

To my way of thinking, for the studio potter, the variety of variables that
influence the usability and appearance of a glaze is so great that fine
details of composition will not, in most cases, make great differences. We
are often encouraging accidental effects in our firing processes, using
variable fuels, adding volatile materials, using chemically active kiln
atmospheres, etc. With this level of variation in all the surrounding
processes, the actual detailed composition of the glaze powder will have to
be relatively low on the list of things making the glaze look the way it
does.

Joseph Herbert

Joseph Herbert
Technical Writer
Irving, Texas
214-725-8305 (Cell)

Ivor and Olive Lewis on wed 5 sep 07

Dear Joseph Herbert,

You say <I have in my mind is something like, "the lowest melting temperature =
composition for a two or more component (material) system.">>

I suggest you test your assertion with the 1170 Eutectic that is =
detailed in Dave Finkelnberg's article in Ceramic Monthly. There is an =
alternative recipe in "Pioneer Pottery" and it can be constructed from =
Quartz, Corundum and Whiting.=20

You will find, if you do a literature search, that a Eutectic is created =
when a molten liquid is in equilibrium with the compounds into which it =
will solidify. Should the temperature rise the solids liquefy. Should =
temperature fall the liquid will solidify.=20

In your Solder example a Lead rich alloy will commence to throw out =
crystals of Lead as the melt cools. This increases the concentration of =
tin in the melt. When the concentration of Tin reaches 62% the liquid =
freezes instantaneously as it cools.

To deal with a four component system K2O-Na2O-Al2O3-SiO2 your diagram =
would be drawn within a Tetrahedron. This allows you to represent =
Composition but not Temperature

That Iron-Carbon system is very complex because it has both Eutectic and =
Eutectoid points.

Best regards,

Ivor Lewis.
Redhill,
South Australia.