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Glaze Slurry is Difficult to Use
We often tend to put so much effort into adjusting our glazes to fine-tune fired properties that we tolerate poor application properties. Such glazes are not only frustrating to use, but they often produce poor fired results. When a slurry is right it should 'gel' and 'hang on'. You should be able to dip your finger in and pull it out with an even coverage and no drips. In fact, a thixotropic glaze will resist shedding off ware covered with wax emulsion! And it will not settle out hard in the container! Yes, there is no reason to put up with a glaze that drips and drips, cracks on drying, dusts and does not produce an even layer (yes, these
problems are all related).
In industry, maintaining the 'rheology' (flow properties) of the glaze slurry through material, water quality, seasonal, and personnel changes is often the most difficult challenge a factory faces. Maintenance of the specific gravity in particular is a reference point, 'an anchor' around which all other adjustments hinge (if your glaze is working well, measure its specific gravity now). If your glazes application or drying properties are often problematic or they are always difficult to work with, read on, recipe change(s) might be most appropriate. Be careful about using glaze additives, try all the other approaches first.
Gel Your Glaze, Adjust its Recipe to Gel
The glaze slurry must be thixotropic, it must "gel" so that the mechanism of its
initial adherence to the ware is, to a considerable extent, a function of this property
rather than absorption of water by porous bisque. While thixotropic behavior can be
achieved by using glaze additives, most people lack the experience, knowledge,
equipment and circumstances to use
them properly. It is thus desirable to avoid additives if possible and try to select a
kaolin or ball clay that contributes thixotropic properties. EPK (kaolin) is a good
example. If your glaze does not contain adequate kaolin (15-20%) then use ceramic calculations
(e.g. Digitalfire INSIGHT) to adjust it so that it does. How is this possible? Because
ceramic chemistry sees materials as 'oxide contributors' and it is thus possible to
supply a specific chemistry from different mixtures of materials.
It is possible to have 20% kaolin in cone 04 glazes
if you use low alumina boron frits. At higher temperatures glazes have significantly more
Al2O3 and SiO2 and so it is usually easy to achieve a 25% kaolin content (because it
contributes Al2O3 and SiO2).
However there are many high temperature glazes that have large percentages of feldspar,
sometimes 70%! In such the feldspar is supplying all of the needed Al2O3 and so there is
little room for clay in the recipe. These glazes are evil and there is no need for this.
The simplest way to fix this problem is use ceramic calculations to reduce the feldspar
and supply the alkali oxides from other sources. This will enable you to increase the kaolin
to supply the lost Al2O3 (from the feldspar reduction). Ferro Frit 3110 is a good example
of a frit that is very similar to a feldspar in chemistry, but it has very low alumina.
Consider What Materials You Are Using
Different clays produce slurries of differing properties. Bentonite-like materials
have the ability to gel in water in small amounts, they will help suspend the other
particles better than any other material. However bentonites gel the water and hold onto it
so well that using any more than 5% will cause glazes dry too slowly and shrink too much. Ball clay
is better, 20% of it in a recipe can produce a nice slurry, and many people prefer its
characteristics. However ball clay glazes do not necessarily gel well (and ball clay
introduces more iron than you might want). If you have a kaolin that suspends well, it is
the ideal material. In North American, EPK, for example, produces very nice slurries that
suspend well and gel to help them hold on immediately after the dip. Experiment with
the kaolins and ball clays available to you to find the best one.
Some materials are soluble or partially soluble, this is even the case
with some frits (which are of course not intended for glazes). When
materials dissolve in the glaze they introduce electrolytes into the water
which in turn can affect the viscosity of the glaze. For example, high
nepheline syenite glazes can thicken over time and each time you add water
to re thin the glaze shrinks and cracks more during drying on the ware. High
boron materials are often soluble. Clays, especially raw and native clays,
often contain soluble sulfates that can dramatically affect the slurry.
These problems can be insidious because these materials often dissolve
slowly overtime and thus the rheology will change accordingly. Admittedly,
companies with a continuous production line can use slightly soluble
materials since their glaze is used quickly and is not stored.
Dense Bisque Ware
Bisque ware should not be too high in porosity. Variable porosity means variable thicknesses in
the overlying glaze. Porous bisque ware demands that glaze slurries be thin and runny or
the application will be too thick. If you are used to bisque firing from cone 010 to 06,
go to 04 or higher if you can and use a more gelled glaze.
Use Additives Only if Absolutely Necessary
Misuse of glaze additives is very common because they are not nearly as well understood
as other materials. Often they are listed in recipes in which they are not really needed. They
should be avoided if possible, because they often have detrimental side effects. Remember,
although you might think your glaze needs them, does it really? Only all-fritted glazes
with very low kaolin normally need additives in typical traditional ceramic applications
(an exception is crystalline glazes that require a low alumina content). Many people use
additives that actually worsen the application properties of their glazes. In
these cases, often a recipe adjustment to increase clay content (by sourcing the same chemistry
from a different set of materials) or a simple bentonite addition would be much
better (i.e. gum additions may give a thinner applying, slower drying, 'drippy'
glaze). Again, do not use an additive if it is not
needed, additives are not a substitute for a good glaze recipe.
Individual additives often defy easy classification because they claim to
impart suspending, adhesive and
flow properties. Thus picking the right one is a matter of discerning the need and using
the additive that 'emphasizes' the needed slurry property and gives the fewest side
effects (i.e. color change, slow drying rate, biodegradation, film formation). I might add
that it is also common to use much more of an additive than is needed, normally completely ruining
glaze slurry properties.
While gum does form a gel to suspend particles, it is usually more useful in making the
slurry 'sticky', and acts as a temporary glue to cement otherwise loosely adhered
particles; thus it is referred to as a 'binder', 'hardener', 'adhesive'. Remember that the
mechanism of glaze adherence is normally simply contact, it 'hangs on' to irregularities
in the surface by virtue of its own strength. Thus a harder dry glaze layer will adhere
better. Note also that clays can impart both dry hardness plus suspension and gelling
properties to the slurry, whereas gums usually only harden it. Starches usually act as
hardeners and may thicken the slurry (therefore suspending it better). Cellulose ethers
are used like gum and starch to harden and thicken, they are said to be more consistent
and easier to control. Claylike plasticizers (like Veegum) can impart similar claylike
properties to a slurry, but remember that the beneficial properties of kaolin, for
example, come largely from having alot of it in the recipe. Bentonite clay, likewise, can
be beneficial but only in amounts small enough that prevent it from slowing down the drying
significantly. So generally clay-like additives have these same limitations. Other
additives include wetting agents, foam control agents and sealers. People who know how to
use these materials can do things with glaze that others might think impossible. Likewise,
those of us who do not know how to use them can create a real mess. Manufacturers usually
have instructions so do not buy these materials without good instructions.
Electrolytes change the pH of the suspension and affect the charge of particles
(this changing slurry viscosity); a few drops can make a thick slurry very runny and thin
(deflocculating it), or make a thin one gel (flocculating it). Thus deflocculants/deflocculants can be
used to adjust otherwise variable flow properties. But this cannot be done by a novice.
The amounts required are generally extremely small and must be tuned to the specific batch
by careful measurements (a few drops too much can literally turn your glaze into jelly
or make it settle like a rock). It is
amazing how much a small amount of a flocculant, such as calcium chloride, epsom salts, or
vinegar can gel a glaze (so it makes sense to test on a small amount before adding it to a
whole batch). At the risk of being repetitive, please consider: if you need to use these
materials is it possible that adjusting the recipe to increase the clay or remove soluble
materials (e.g. boric acid, nepheline syenite, lithium carbonate) would be a better
approach. One real downside of these materials is they can put a glaze batch on a
roller coaster viscosity ride, even with powerful mixing equipment to try stabilize their
action. Do you really want that?
If you are storing your glaze slurries it is very beneficial to have a mixer
that can put alot of energy into the slurry to thoroughly wet the surfaces of
all particles during primary mixing. After this, final adjustments with water
content and possible additives can be done to establish the final rheological
properties. When this is done the glaze slurry will be more stable for a longer
period of time.
My general advice is this: If your glaze is not suspending, hardening, gelling
properly, then, if possible reformulate it to have more clay, especially kaolin. If it still
needs help then add bentonite (up to 3%). If the glaze still needs extra
help, then use an additive, but beware. If there are still problems, then, heaven forbid,
use a flocculant or deflocculant or study up on other more exotic additives!
The ball clay you use to suspend your glaze is important!
I poured 4 teaspoons of two glazes onto a board and let them sit for a minute, then inclined the board. The one with Gleason Ball clay (right, much higher in coal and finer particle size) has settled and the water on the top of running off. The one with Old Hickory #5 ball clay has not settled at all and the whole thing is running downward. Below I have begun to sponge them off. Old Hickory No. 1 Glaze Clay is even better than #5 for suspension. The most amazing thing about this: There is only 7% ball clay in the recipe.
Out Bound Links
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