目
录
典型陶瓷制品釉料配方列表
熔块概述
Huge quantities and varieties of
frits are manufactured for the ceramic industry every year by dozens of
different companies. They are made by melting mixes of raw materials in special
kilns, then pouring the mix into water and finally grinding it into a fine
powder.
Frit suppliers refer to the use of
their frits in 'partially fritted' and 'all-fritted' glazes. The latter
generally refers to glazes with 90% or more frit, the former to 90%.
Although the fritting process is
expensive there are many advantages to using frits in glazes, enamels, etc.
-To render soluble materials
insoluble
Often very useful oxides (i.e. boron) are contained in high proportions in raw
materials that are either slightly or very soluble. These normally cannot be
used in glazes because they have adverse effects on the slurry's fluidity,
viscosity, thixotropy, or make it difficult to achieve or maintain the desired
specific gravity. In addition soluble compounds are absorbed into porous bodies
during glazing and this compromises the body's resistance to bloating and
warping and the glaze's homogeneous structure. Fritted mixes containing these
materials renders them insoluble and inert.
-To improve process safety of toxic
metals
Some materials contain undesirable and unsafe compounds. The fritting process
drives these off. Many other materials are unsafe in the workplace and fritting
decreases their toxicity for ceramic production workers. Lead is a prime
example. Lead frits decrease the process toxicity of raw lead compounds. Barium
is another example. However the normal fritting process has no effect on whether
or not a fired glaze will leach or not. This is a function of its chemistry,
unbalanced and unstable glaze formulas are just as likely with frits as without.
The primary safety benefit for frits is thus for workers who use frits in
manufacturing.
-To reduce melting temperature and
improve melt predictability
Since frits have been premelted to form a glass, remelting them requires less
energy and lower temperatures. Frits soften over a range of temperatures (in
contrast to crystalline raw materials that melt suddenly) and lend themselves
very well to production situations where repeatability and ease-of-use are
necessary.
-To avoid volatilization of unstable
substances
Most raw ceramic materials contain sulfur or carbon compounds as well as H2O.
These vaporize at various temperatures as materials decompose and are driven off
as gases during firing. This volatilization activity has a detrimental effect on
the glaze surface and matrix. The fritting process drives off these compounds
and glazes are thus much more defect free. Barium and strontium frits are good
examples, barium and strontium carbonate produce a lot of glaze defects because
of the gases of decomposition they produce.
-To achieve homogeneity
Other than dissolution and very localized migration, fired raw glaze melts do
not mix well to create an evenly dispersed oxide structure. The fritting process
employs mechanical mixing to assure a completely homogenous glass that will
exhibit the intended properties.
-To achieve oxide blends that are
difficult or impossible with raw materials.
Many glaze formulations cannot be achieved with insoluble raw materials (i.e.
high borax, high sodium). Frits employ soluble materials to make almost any
combination possible. One interesting group is the 'specific oxide'
borosilicates, they contain borosilicate and one other oxide (i.e. calcium,
barium, sodium, strontium, lithium). Frits GF-125, 129, 143, 154, 156 are
examples.
-Improve the quality of decoration
Over and underglaze colors work better with frits than raw materials because the
former are cleaner, less reactive, melt evenly, and have a more closely
controlled chemistry. This means colors are brighter by virtue of compatible
chemistry, by better glaze clarity. Edges of colors also tend to bleed less and
color quality is homogeneous rather than variegated (although variegating
materials can be introduced to introduce this quality if desired).
-Fast fire technology
Industry now measures firing time in minutes instead of hours. Frits can be
formulated to melt quickly and completely after body gases have been expelled,
thus greatly reducing glaze imperfections.
-Matte glazes
High zinc, barium, calcium, strontium, and alumina frits can be used and blended
to create quality matte glazes that are very difficult to make with raw
materials that do not melt enough and produce gases of decomposition.
-Opaque glazes
When zircon is added to a frit during the smelting process it is a more
effective opacifier. Clear and opaque frits can be blended to give excellent
control over opacity.
In recent years frits are even being
used at high temperatures. For example defect free high strontium, barium and
even calcium glazes can be made more easily with added frit, especially in fast
fire operations.
The Frit market is driven by large
customers who need certain formulations and by the prepared glaze industry.
Availability of smaller quantities of frits are generally determined by what
industry is using. Since the frit market changes with time, so does the
availability of some frit types.
In recent years some frit companies,
such as Fusion Ceramics, freely supplied the chemical analysis of their frits.
Others, such as Ferro who were forthcoming with data in the past, have been more
guarded and either provide no chemistry or approximate analyses. There is now a
situation of 'legal chill' in the industry because of law suits against the frit
manufacturers for deviance of their products from the published formulas. Now
most companies are hesitant to supply formula data. Since most standard frits
have been made for many years users are doing 'intelligence work' to get the
formulas by finding them in older magazines and books and by deducing them from
charts of equivalent frits from different manufacturers.
However each manufacturer makes
specialized frits (i.e. strontium, lithium compounds) that they invest heavily
in R&D to develop. The makeup of these are usually kept secret to protect
against the formulations be copied by other manufacturers. Even though powdered
samples of these frits could be analysed by competitors to deduce their
approximate makeup, the tightly controlled chemistry required to achieve the
intended effect may not be evident. Thus the actual production of a duplicate
can be a more elusive goal than it at first seems.
These factors are of great interest
to people using ceramic calculations. The secrecy makes little sense since it
partially defeats the whole purpose of using frits, namely, having control. It
also works against the general trend toward using the oxide viewpoint to take
greater control of glaze properties.
By copied
http://www.crramic.com
如何调整釉料的烧成温度？
Just as many people are not what they claim to be, glazes
often bill
themselves as one thing yet prove to be quite another. A glaze
recipe might claim to be for cone 6, for example. However, that
label could well be strictly a matter of opinion! The truth is,
glazes do not "melt" at one particular temperature, rather they
'soften' over a wide range. The glaze mentioned above may begin to
melt at cone 02; but someone has chosen cone 6 to "freeze in" the
developing melt for reasons specific to their circumstances and
tastes. The rate of rise, total firing time, and rate of cool are
all critical factors specific to individual situations. Thus, the
main thing determining a glazes firing temperature is you. In fact,
you are also the potential controller of that softening range (I
will refer to temperature according to Orton cone numbers)."A
crystalline material has a specific melting temperature..."
There are factors that ma e judging the firing range of a
glaze much more
complicated than it might at first seem. A crystalline material has a specific
melting temperature at which the regular lattice structure is broken. A glass,
on the other hand, has a much more random molecular structure (called
amorphous). Thus, the bonds holding it together vary in their strength to resist increasing
temperature. The result is that a glass melts gradually over a
sometimes wide temperature range. This is called "softening". Glaze
recipes are
made from powdered materials that are amorphous (e.g. frit), crystalline
(flint), or both (e.g. feldspar). Each behaves very differently as temperature
increases. A recipe that contains all of them will thus have a very complex
melting process. An Orton cone itself is a fine example (it softens and bends
many cones before it fully melts).
Individual materials in a recipe, typically, have a very wide
diversity of
particle sizes. In spite of what textbooks say about the development of
eutectics, these theoretical processes occur predictably only with extremely
fine and well-mixed ceramic powders. The materials having finer sizes will melt
much more quickly. For example, visualize melting a large chunk of wax. Now
grate it into small flakes and melt them. I think you see what I mean. Likewise, finer
materials in the glaze will begin to melt long before others. As this
occurs, they react with the others whose melting, in turn, accelerates. It is
possible to stop a firing during this process and solidify whatever has
occurred. Is such a glaze "mature?" To answer, consider that the rate
of
increase in temperature can significantly influence melting activity. Given more time,
many more interparticle reactions will take place. Given less time and higher
temperature, those components of the glaze having a lower melting
temperature will become much more fluid and react less with other particles. In
an extreme case, the former will produce a well-developed glaze; the latter a
matrix of unmelted particles barely held together by a glassy glue. Of course,
you want to use your fluxes as molecular building blocks in a glaze, not as
interparticle glue. So for a glaze to be mature, one would expect that at least
enough temperature and time has been applied to melt most of the mass. At a
minimum, it should be a very stiff melt. If a glaze lacks lustre and hardness,
then it is possible it is being frozen at this early stage. Then again, maybe
that's what you want; so for you the glaze is mature. "...a glass
melts gradually over a sometimes wide temperature range". The greater
melt fluidity provided by more time and temperature in the kiln affords more
molecular mobility. Given sufficient freedom to move, the molecules will arrange
themselves in an increasingly preferred matrix and cooling will freeze this as a
solid. Depending on the above and other factors, the temperature range over
which this occurs can be surprisingly wide.
So, where should it be halted? The simple answer is: At the
body's optimal
maturing point. As a glaze becomes more and more fluid, it begins to react with
the body to form an interface of layers of intermediate compositions. The better this
interface develops, the stronger and more functional the ware will be. So, as
you can see, an ideal clay-glaze combination is one where both reach optimal temperature
at the same time. Remember, also, that optimal body temperature is not
always the temperature of highest fired strength. It is more likely to be a temperature
of compromise between fired strength and resistance to warp in the kiln; a
'window' within which a particular visual effect occurs or simply the lowest
practical temperature at which a target strength is achieved. So, what is ideal
to produce a mature glaze? Only you can say but it appears it is important to
have some control to optimize a glaze to the body's ideal.
Since the body and glaze themselves produce gases of
decomposition during
development, the fluid glaze has to pass these. Early in the fluid cycle, these
bubbles are abundant and as thermal soaking or temperature rise continue, they
have opportunity to work their way out and break at the surface. At what point
is the glaze mature? Only you can say.
Once a glaze has melted completely and established free
movement of oxide
molecules, another process is under way. The fluid melt is stiff and viscous at
first, but as temperature increases, it thins and eventually will either run off the
ware, boil, volatilize, soak into the body, or react strenuously with it. Interestingly,
if you slow-cool a glaze fired well into this stage, it can
produce very matte surfaces. This is because a network of very fine crystals
develops at the surface. The slower the cool and the higher the temperature, the better
the development. One other factor is worth noting. Some fluxes, like MgO, act
as refractories until a specific temperature where they suddenly flow and become
active. Glazes frozen in this range have a distinctive mottled effect
resulting from the uncombined MgO actively flowing and 'feathering' itself
throughout the viscous melt.
Thus, there is no simple answer to what temperature a
particular glaze fires
best at. There are so many materials, process, and finished product
considerations that it is impossible to make any rules, except one: Flow and
tile test your glazes at many temperatures so that you know what you have. I
have another suggestion also: Don't use too many recipes. Strive toward using a
few, or maybe variations on just one. In this way, you will be able to study and know
the glaze(s) you use and then be in a position to adjust and control them. It
is better to be on friendly terms with one than enemies with twenty.
A Glaze Flow Testing Device
There are many instruments designed to observe and measure
the thermal
expansion, melting temperature, hardness, and strength of fired glazes. And
now,NDT (non-destructive testing) equipment can test many ceramic parts without
even damaging them. Most of us just do not have access to such equipment
and have to depend largely on simple observation techniques. I would like
to submit a device as a general purpose tester that provides plenty of
bang-for-the-buck. It is easy to make and use, it is inexpensive, and
provides much more information than you might think.
We all know that a glaze's job is to melt and form a smooth
glassy surface on a clay substrate. So a test that tells us how glassy, how
smooth, and how melted would seem valuable.
Before going on, I will give credit where credit is due. This
is not an original idea. I have seen this device described in industry
literature. They use it to compare melt properties related to the particle size
of nepheline syenite. Also, I was sent a very nice dual-flow mold by Hugh Nile
at Sterling China. I am aware that other industries also use similar devices.
The Small Tester
I have found an ideal size for the smaller type to be about 3
cm high. This
allows a large enough reservoir and long enough ramp for rough tests and is
still small enough to allow gluing the tests on boards for record keeping. The
dam at the bottom catches overflow so it does not stick to the shelf. It is best made
by simply pressing plastic clay into a plaster mold from the back. Dry and bisque
fire the samples, then fill the F( H` d reservoir with glaze by smearing it
in as a paste.
The Dual-Flow Large Tester
This one works much better in a large 12-13cm size. The long
runway provides for quite a sensitive test and even small differences in flow
properties of two
samples are easily recognized. The rocking bottom allows you to set it at a
steep or shallow angle. This device should be cast in a plaster mold using a
slurry body of fired properties similar to the production material. Dry and
bisque fire the testers, then dewater and roll a specimen of the material to be
tested into a ball and press it into the reservoir.
What You Learn
During the firing, the glaze flows down the runway according
to the
melt development, melt surface tension, and internal turbulence.
"...a test that tells us how glassy, how smooth, and how melted
would seem valuable."
For consistent results, it is important that your testers be
the same thickness, set at the same angle, made from the same clay, and
fired to the same
temperature each time. To verify, place a cone or bullers ring right beside each one.
One of the big advantages of the dual tester is that for comparative
purposes in testing two specimens or one alongside a benchmark, temperature
control is not as important.
The following information about a glaze is then evident:
The degree of fluidity of the melt. Glazes applied to tiles or ware that
appear similar will often display drastically different melt fluidities on a
flow tester.
Quality control: If glaze ingredients shift in particle size
or chemistry
and thus change the melt, it will be immediately evident either by the flow
reaching further down the runway or a change in the character of the flow.
Information from a flow tester is valuable in adjusting the recipe of a
glaze while maintaining the same fluidity.
A flow test can be valuable for evaluating basic mechanisms
in glazes. For
example, it can be used to check individual raw materials, either pure or
blended, with reference materials (e.g. mix flux with flint or kaolin). For
example, consider a stony matte. Is it matte because of immaturity or super
saturation in a fluid glaze? This test will tell you right away.
Ball milling time: By extracting samples from your mill at
regular
intervals, firing, and comparing the degree of flow, you will be able to
assess the mill's effect on glaze maturity and melt development.
The number of bubbles in the melt: Because the glaze is so
thick, any
bubbles resulting from products of decomposition within the glaze will be
evident by the character of the flow and in the broken cross section
(bubbles will often disrupt the melt flow). This is because there is too
great a distance for most bubbles to migrate to the surface.
Changes in other properties like crystal development,
tendency to crawl,
blister or boil, opacity and thermal expansion are amplified by a flow test.
This device can even be used to help determine the optimal
firing
temperature of a glaze and the range at which the fluidity begins to change
more quickly. After all, what is more significant to determine the
freeze-point than flow of the glaze melt?
Changing a Glaze's Firing Temperature
There is ample economic and environmental incentive in both hobby and industrial
circles to fire kilns to the lowest temperature possible. However, there is
understandable resistance to changing existing formulations that have been
working. Body recipes might also need adjustment, since they could lose strength
and density in the process. On the other hand, changes in body materials may
mean it is desirable to fire a little lower. Admittedly, in some cases it is
actually better to fire higher, since lower temperatures sometimes require more
expensive and highly processed materials, many of which are environmentally
hostile. They also require more testing and control, and lower temperatures
are not as forgiving. In addition, some glazes depend on a melting or
freezing behavior that is specific to a narrow range of temperatures (i.e.
MgO). "...it is frequently possible to achieve equal or even greater
strength in
body and glaze at a lower temperature."
Still, using proper preparation, it is frequently possible to
achieve equal or even greater strength in body and glaze at a lower
temperature. Sometimes just moving one cone lower can mean significantly
less stress on the kiln and an
appreciable reduction in fuel consumption.
Most people don't realize how easy it can be to adjust the
average balanced
glaze to a new temperature. Let's look at two formulation approaches to this
problem. Since these depend heavily on your ability to do many quick
recipe-to-formula calculations, INSIGHT is indispensable.
Before attempting to reformulate a glaze to a lower fire,
consider if it is
necessary. If the glaze to be adjusted is a glossy or matte base with opacifier
or colorant added, then do you already have a similar base at the lower
temperature? Will it work with the same additives as is or with minimal
adjustment? If so, then use it. If not, then let us continue.
First, what is different about formulations for high and
low-temperature glazes?
Here is an example of approximate oxide ranges for Orton cone
6 and 10 lead-free standard whiteware and pottery glazes (the figures to
follow compare the numbers of oxide molecules of each assuming flux unity).
Cone 6 Cone 10
CaO ........0.2-0.5 0.4-0.7
ZnO ........0.1-0.3 0-0.3
BaO ........0.1-0.3 0-0.3
MgO ..........0-0.2 0-0.4
KNaO....... 0.1-0.3 0.2-0.4
Al2O3.......0.2-0.3 0.3-0.5
B2O3 .......0.3-0.6 0.1-0.3
SiO2......... 2-3 3-5
It is the amount of SiO2, B2O3 and Al2O3 whose proportions
really determine the melting temperature of a glaze. Also fluxes are more
diversified. Notice that the Al2O3 and SiO2 are about one third less for
cone 6 than cone 10. Also B2O3 materials can melt as low at cone 06, so
increasing it could be a real help to reduce a glaze's firing temperature.
The glaze that I want to adjust is a cone 10 reduction
celadon, and I wish to bring it down to cone 6R. This is quite an ambitious
undertaking. Although cone 6 and cone 10 may not seem far apart on paper,
there is usually a vast
difference between clays and glazes intended for each. To be honest, there are
many recipes that cannot be converted without the introduction of more active
fluxes that can change the visual character. Recipes that have abundant kaolin,
ball clay, feldspar, and flint are often prime candidates for change. On the
other hand, as already mentioned, don't convert a recipe if the same base type
is already available at the lower temperature. In this case if you had a
celadon-like stiff-melt clear that suspends micro-bubbles and reacts with iron
to give green, then no conversion would be necessary.
Here is a detail calculation of the glaze I want to change
DETAIL PRINT - Cone 10R Celadon
MATERIAL PARTS WEIGHT CaO* MgO* K2O* Na2O* Fe2O3* Al2O3 SiO2
WEIGHT OF EACH OXIDE 56.1 40.3 94.2 62.0 160.0 102.0 60.1
---------------------------------------------------------------------------
Expan OF EACH OXIDE 0.15 0.03 0.33 0.39 0.13 0.06 0.04 Cost/kg
-------------------------------------------------------------------------
CUSTER FELDSPAR.... 25.50 617.10 0.00 0.03 0.01 0.00 0.04 0.29 0.00
WHITING............ 14.00 100.00 0.14 0.12
KAOLIN............. 19.00 258.14 0.07 0.15 0.24
FLINT.............. 31.00 60.00 0.52 0.19
IRON OXIDE RED....... 4.00 160.00 0.03 2.90
DOLOMITE..............8.50 184.00 0.05 0.05 0.00
---------------------------------------------------------------------------
TOTAL ..............102.00 0.19 0.05 0.03 0.01 0.03 0.12 0.96 0.23
UNITY FORMULA........ 0.63 0.15 0.09 0.04 0.09 0.39 3.20
PER CENT BY WEIGHT ..11.79 2.09 2.88 0.86 4.56 13.33 64.49
Cost/kg 0.23
Si:Al 8.21
SiB:Al 8.21
Expan 6.75
Notice the Al2O3 is in the middle of the normal range for
cone 10 glazes and the SiO2 is at the low end of its range (the 'Unity
Formula' line). To adjust this recipe to cone 6, the strategy will be
simple: put the Al2O3 in the middle of the cone 6 range and the SiO2 at the
low end. I will be retaining the SiO2 :
Al2O3 ratio at around 8.0 and won't be touching the proportions of any other
oxides, so the appearance of the glaze should be retained.
To reduce Al2O3 and SiO2 , first reduce materials
contributing them. From the above printout, notice that kaolin and flint
contribute both and contain no
other oxides. Had this recipe lacked kaolin, I would have had to reduce the
feldspar to cut Al2O3 and compensated for the loss of other oxides it
contributed.
Following is a detailed calculation after the reductions were
made
DETAIL PRINT - Cone 6R Celadon
MATERIAL PARTS WEIGHT CaO* MgO* K2O* Na2O* Fe2O3* Al2O3 SiO2
WEIGHT OF EACH OXIDE 56.1 40.3 94.2 62.0 160.0 102.0 60.1
---------------------------------------------------------------------------
Expan OF EACH OXIDE 0.15 0.03 0.33 0.39 0.13 0.06 0.04 Cost/kg
---------------------------------------------------------------------------
CUSTER FELDSPAR.... 33.00 617.10 0.00 0.04 0.02 0.00 0.06 0.38 0.00
WHITING............ 18.00 100.00 0.18 0.12
KAOLIN............. 11.50 258.14 0.04 0.09 0.24
FLINT.............. 22.50 60.00 0.38 0.19
IRON OXIDE RED...... 4.00 160.00 0.03 2.90
DOLOMITE........... 11.00 184.00 0.06 0.06 0.00
----------------------------------------------------------------------------
TOTAL............. 100.00 0.24 0.06 0.04 0.02 0.03 0.10 0.84 0.21
UNITY FORMULA .......0.64 0.16 0.09 0.04 0.07 0.26 2.23
PER CENT BY WEIGHT. 15.88 2.82 3.90 1.17 4.79 11.98 59.46
Cost/kg 0.21
Si:Al 8.42
SiB:Al 8.42
Expan 7.60
When doing this type of adjustment, keep a few things in
mind.
Kaolin was used in the original recipe instead of ground
alumina to
source Al2O3 and for good reason. Not only is it inexpensive but it
acts as a suspender to keep particulates from settling. Reducing it
to accomplish a reduction in Al2O3 is fine but to retain reasonable
suspension sometimes you may have to add bentonite or switch to the
more effective ball clay to supply the reduced Al2O3 quota. Also, if
the recipe total changes, remember to maintain the same percentage
of iron oxide (or other colorants, opacifiers, etc.) in the recipe.
'Kaolin is an ideal source of Al2O3 ...not only is it inexpensive
but it acts as a suspender to keep particulates from settling'
Notice the calculated expansion has increased because of a
reduction in the two oxides, which make the greatest contribution to
keeping it low. This means there is a chance the glaze may tend to craze.
Since middle temperature glazes have less SiO2 and Al2O3 , crazing is more
common anyway. If glaze fit proves to be a problem, we could probably
increase the SiO2 without adverse effects, some
higher-expansion CaO could be exchanged for some lower expansion MgO, or B2O3
could be introduced at the expense of some of the high-expansion Na2O and K2O.
So far, I have just done a calculation and hoped for the
best. An auxiliary
approach is to make a line blend.
To do this I will calculate the kaolin to flint mix, which
has the same SiO2 :Al2O3 ratio as the original glaze will remove the two
materials from the
formula in this ratio.
Shown here is a partial report of the results of a
calculation to determine the kaolin:flint mix to yield a SiO2 :Al2O3 ratio
of 8:1.
FLINT/KAOLIN MIX TO YIELD
SILICA:ALUMINA RATIO OF 8:1
---------------------------
KAOLIN........... 5.00
FLINT............ 7.00
Al2O3 .19 17.49%
SiO2 1.55 82.51%
Rather than mixing and weighing out a test batch of each
blend, there is a
simpler way. Mix up a test using 10 less kaolin and 14 less flint (the
proportion just determined).
Adjust the recipe so that you retain the 4% iron; it should
work out like this. 102
ORIGINAL ALTERED Total
POTASH FELDSPAR 25.50 25.5 34.0
WHITING........ 14.00 14.0 18.5
KAOLIN......... 19.00 9.0 12.0
FLINT.......... 31.00 17.0 22.5
IRON OXIDE.......4.00 4.0 4.0
DOLOMITE.........8.50 8.5 11.0
------ ----- ------
102.00 78.0 102.0
Now, line blend this 75:25, 50:50, and 25:75 with the
original. There is an easy way to do such a blend:
Set out 5 foam cups labeled A-E.
Obtain a veterinarian's 60 cc syringe (they are inexpensive
and easy to
get).
Draw up 100 cc of the original, and expel it into cup A.
Draw up 100 cc of the test mix and squirt it into cup E.
Draw 50 cc of the original and 50 cc of the test and put them
into cup C.
For cup B, take 75 of the original and 25 of the test, and
for cup D, 25 &
75.
Stir the mixes using a spoon, dip your samples for both
vertical and
horizontal placement in the kiln, and fire.
This method is very quick and line blends of 10 intervals are almost as simple
to do as those having 4.
We have done only one glaze here but the technique is quite
simple. This method assumes that the original glaze is not an unbalanced or
critical eutectic
mixture. Certainly, there are other ways to optimize melt temperature in a
glaze. Sometimes you can do it by moving to a nearby eutectic mixture (with the
help of the dreaded phase diagram). Also, the miracle oxide of low-temperature
glazes, B2O3 , is always available to move a glaze down while maintaining its
expansion. Small amounts of powerful fluxes like lithium or zinc can sometimes
help. I leave it to you to explore some of these avenues. But for heaven's
sake,don't just blindly throw in a frit.
One final thing: You need to evaluate the new lower melting
glaze. The best
approach is to use a flow tester to compare melt flow at the new temperature
with flow of the original at the old.
Copyright 1996 Author: Tony Hansen
通过计算调整釉料的膨胀系数
Shivering and crazing are probably the two most common
glazing problems in industry and hobby ceramics. We have already talked a lot
(and will talk more) about the theory of thermal expansion, and once you become
aware of how easily this is dealt with, it becomes a mystery why others seem
puzzled by the problem. I have watched people who are in love with one touchy
glaze try a myriad of different bodies, alter firing curves in every possible
way, and yet, achieve only limited success. And then, there are the "snake
oil" remedies. "Just add some flint to stop crazing and take some out
to stop shivering" it says in many textbooks. This sounds fine at first but
as already discussed, in solving the problem, you will likely upset the oxide
balance and probably kill its surface character. Remember, each glaze has its
own personality and complexities, and the "one solution fits all"
theory will not work.
Until now, calculating glaze formulas and their expansion
values has been a
little tedious to say the least. As we have seen, INSIGHT and similar software
changed that, providing instant expansion results for any formula or recipe.
Below is a set of source figures I have chosen to use. As you will see, it is
not necessary to know what a glaze's expansion is; all we need to know is
whether one glaze has a higher expansion than another. The units of these
numbers have already been discussed and are not important. Just remember that an
expansion of 6 is higher than 5.
Expansion Values BaO 0.13 K2O 0.33 MnO -0.09 CaO 0.15 Na2O
0.39 TiO20.14 PbO 0.08 ZnO 0.09 B2O3 0.03 MgO 0.03 Fe2O3 0.13 Al2O3 0.06 SiO2
0.04
From the numbers above, it becomes obvious why the addition of flint has a
lowering effect on a glaze's expansion. One also begins to appreciate why
high-temperature glazes, which have much more SiO2 and Al2O3 , are so much
easier to fit to clay bodies without crazing. SiO2 has a very low expansion, so
glazes containing plenty of it will tend to have a low expansion also and thus
resist crazing (and move toward shivering).
Let's look at a shivering problem that occurred in one
studio. Below is an
INSIGHT report of the original shivering glaze.
DETAIL PRINT - Matte White Glaze
MATERIAL PARTS WEIGHT CaO* MgO* K2O* Na2O* ZrO2 Fe2O3* TiO2 Al2O3 SiO2
WEIGHT OF EACH OXIDE 56.1 40.3 94.2 62.0 123.2 160.0 79.7 102.0 60.1
-------------------------------------------------------------------------------
MATERIAL
Expan OF EACH OXIDE 0.15 0.03 0.33 0.39 0.02 0.13 0.14 0.06 0.04 Cost/kg
------------------------------------------------------------------------------
长石............... 52.50 617.10 0.00 0.06 0.03 0.00 0.09 0.60 0.00
苏州土............. 15.00 258.14 0.06 0.12 0.24
石英............... 14.50 60.00 0.24 0.19
钟乳石............. 9.50 100.00 0.09 0.12
滑石............... 5.00 126.23 0.04 0.05 0.25
锆英砂............. 1.50 183.00 0.01 0.01 0.00
BENTONITE.......... 2.00 489.48 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00
------------------------------------------------------------------------------
TOTAL ............100.00 0.10 0.04 0.06 0.03 0.01 0.00 0.00 0.15 1.04 0.09
UNITY FORMULA...... 0.44 0.18 0.26 0.11 0.04 0.01 0.00 0.68 4.70
重量百分比......... 5.87 1.76 5.72 1.69 1.08 0.22 0.00 16.45 67.20
Cost/kg 0.09
Si:Al 6.93
SiB:Al 6.93
膨胀系数 6.90
Notice that the expansion is calculated based on the figures
shown under each oxide column title. It is very clear which oxides need to
be increased to move the expansion up. Shivering is much less common than
crazing, so it is not
likely that too much change is needed. It should be possible to leave the SiO2
and Al2O3 alone and thereby minimize fired property changes associated with
disruptions in the overall balance.
If exotic color compatibility is not at issue (it depends on
the absence or
presence of certain fluxes), a conservative starting point is to redistribute
the fluxes, increasing one at the expense of another. Sodium has the highest
expansion, so it is logical to increase it at the expense of others (in this
case CaO and K2O). You have probably noticed that high sodium glazes (ones with
nepheline syenite and soda feldspar) tend to craze. This phenomenon is at the
opposite end of the glaze fit scale, we need to move toward it.
A common sodium sourcing material, as mentioned, is nepheline
syenite. Using Glaze Tools, I made a quick substitution for potash
feldspar, then juggled remaining material amounts to compensate for the
differences in these two materials. Here is the result.
DETAIL PRINT - Matte White Glaze
MATERIAL PARTS WEIGHT CaO MgO K2O Na2O ZrO2 Fe2O3 TiO2 Al2O3 SiO2
WEIGHT OF EACH OXIDE 56.1 40.3 94.2 62.0 123.2 160.0 79.7 102.0 60.1
------------------------------------------------------------------------------
Expan OF EACH OXIDE 0.15 0.03 0.33 0.39 0.02 0.13 0.14 0.06 0.04 Cost/kg
------------------------------------------------------------------------------
NEPHELINE SYENITE... 49.30 446.40 0.01 0.00 0.02 0.08 0.11 0.50 0.33
KAOLIN............. .10.81 258.14 0.04 0.08 0.24
FLINT.............. .22.70 60.00 0.38 0.19
WHITING............. 9.00 100.00 0.09 0.12
TALC................ 4.60 126.23 0.04 0.05 0.25
SUPERPAX............ 1.50 183.00 0.01 0.01 0.00
BENTONITE........... 2.00 489.48 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00
------------------------------------------------------------------------------
TOTAL ..............99.92 0.10 0.04 0.02 0.08 0.01 0.00 0.00 0.16 1.04 0.25
UNITY FORMULA....... 0.10 0.04 0.02 0.08 0.01 0.00 0.00 0.16 1.04
PER CENT BY WEIGHT ..5.74 1.66 2.49 5.18 1.07 0.08 0.00 17.41 66.37
Cost/kg 0.25
Si:Al 6.47
SiB:Al 6.47
Expan 7.17
Notice that the calculated expansion is up modestly to 7.2
(from about 7.0). The relative amounts of the fluxing oxides (refer to the
"UNITY FORMULA" line) have altered somewhat, but the SiO2 and
Al2O3 remain unchanged as a result of the compensatory recipe adjustments
done during calculation.
It is possible that this change in expansion might not be
enough. There is still room for more movement here but it is possible that
the glaze depends partly on the magnesia (MgO) for its silky appearance.
Following is a calculation where I have eliminated the MgO
sourced by talc, and added CaO.
DETAIL PRINT - Matte White Glaze
MATERIAL PARTS WEIGHT CaO* MgO* K2O* Na2O* ZrO2 Fe2O3* TiO2 Al2O3 SiO2
WEIGHT OF EACH OXIDE 56.1 40.3 94.2 62.0 123.2 160.0 79.7 102.0 60.1
----------------------------------------------------------------------------
Expan OF EACH OXIDE 0.15 0.03 0.33 0.39 0.02 0.13 0.14 0.06 0.04 Cost/kg
---------------------------------------------------------------------------
NEPHELINE SYENITE... 47.71 446.40 0.01 0.00 0.02 0.08 0.11 0.48 0.33
KAOLIN............. .10.75 258.14 0.04 0.08 0.24
FLINT............... 25.42 60.00 0.42 0.19
WHITING............ .12.61 100.00 0.13 0.12
SUPERPAX............ 1.50 183.00 0.01 0.01 0.00
BENTONITE.......... .2.00 489.48 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00
-----------------------------------------------------------------------------
TOTAL.............. 99.99 0.13 0.00 0.02 0.08 0.01 0.00 0.00 0.16 1.02 0.25
UNITY FORMULA .......0.56 0.01 0.10 0.32 0.03 0.00 0.00 0.67 4.34
PER CENT BY WEIGHT ..7.98 0.10 2.44 5.08 1.09 0.08 0.00 17.22 66.01
Cost/kg 0.25
Si:Al 6.51
SiB:Al 6.51
Expan 7.39
As you can see, the expansion has increased much more this
time; possibly it was not necessary to remove all the talc. A line blend of
this adjustment with the original recipe would determine some intermediate
compromise mixture. A simple three-interval line blend is done by mixing
the trial 75:25, 50:50 and 25:75 with the original. An easy way to do this
is described in the section about altering glaze temperature.
I am going to try one more change.
I will go back to the original recipe again, leave the potash
feldspar alone, remove the talc, and adjust the rest of the recipe
ingredients to preserve the SiO2 : Al2O3 ratio.
DETAIL PRINT - Matte White Glaze
MATERIAL PARTS WEIGHT CaO* MgO* K2O* Na2O* ZrO2 Fe2O3* TiO2 Al2O3 SiO2
WEIGHT OF EACH OXIDE 56.1 40.3 94.2 62.0 123.2 160.0 79.7 102.0 60.1
------------------------------------------------------------------------------
Expan OF EACH OXIDE 0.15 0.03 0.33 0.39 0.02 0.13 0.14 0.06 0.04
Cost/kg
------------------------------------------------------------------------------
CUSTER FELDSPAR..... 49.75 617.10 0.00 0.05 0.02 0.00 0.08 0.57 0.00
KAOLIN............. .16.50 258.14 0.06 0.13 0.24
FLINT............... 15.75 60.00 0.26 0.19
WHITING............. 14.50 100.00 0.14 0.12
SUPERPAX............. 1.50 183.00 0.01 0.01 0.00
BENTONITE............ 2.00 489.48 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00
--------------------------------------------------------------------------------
TOTAL............. 100.00 0.15 0.00 0.05 0.02 0.01 0.00 0.00 0.15 0.99
0.09
UNITY FORMULA ..... .0.65 0.01 0.24 0.11 0.04 0.01 0.00 0.67 4.35
PER CENT BY WEIGHT- 9.07 0.05 5.56 1.64 1.11 0.22 0.00 16.97 65.37
Cost/kg 0.09
Si:Al 6.54
SiB:Al 6.54
Expan 7.23
The expansion has moved from 7.0, for the original, to 7.3
with this modest
change. Again, it is possible that the optimum recipe is a mixture of this and
the original as determined by a line blend.
Notice that throughout this calculation process, I kept the
total recipe amount at 100. Also, I rounded recipe amounts on each
calculation to avoid accuracy overkill. Since the Superpax and bentonite
are added for non-chemical purposes of melt opacity and slurry suspension,
as a final step I recalculated the recipe total to 96.5 and restored the
superpax to 1.5 and the bentonite to 2.0.
You probably want to know whether this really worked! Well,
it performed
perfectly! All three trials had a surface quality that is almost identical to
the original. The first two high sodium tests made the glaze a little less
opaque, requiring the addition of more Superpax. A little work with the three in production
indicated the best one.
Crazing is equally easy to handle, just reverse the above
process. However,
since crazing is so much more common, it may be necessary to make greater
changes to the recipe.
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Copyright 1996 Author: Tony Hansen
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