Pottery and Ceramics Glaze
Surface Tension and Glazing
Well, below is some technical information about putting glaze on bisque. This is important, because if you don't have a bisque - glaze combination that expands and contracts at the same rate, you are going to have a bunch of cracks on your pottery piece. Also, if you get to much glaze on a piece it will run and if there isn't enough there will be light colored areas showing. I have taken out the hundred dollar words and replaced them with common ordinary language that everybody can understand.
Sometimes the crack can be a planned outcome.
Curvature can be used to control crack formation, as demonstrated here. Notice the high concentration of cracks around areas of high curvature, such as the ears, neck and leg. The image on the right was generated using only uniform shrinkage of the surface, resulting in more evenly spaced cracks.
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Surface Tension
Surface tension represents the force of the film on the liquid surface.
When one applies this principle to a mineral suspension, one can see that the surface tension of water creates on the surface, a layer which contains the finest particles of the suspension.
This layer is very thin and created without apparent external cause.
Application to an isolated drop of the suspension.
A drop is a volume of insulated liquid where sticking together is ensured by the forces of surface tension.
In the absence of external influence, the drops have a round form.
If you were to apply this reasoning to a drop of this suspension, the finest particles come to cover the surface of this one and leave in the center a lot of suspension richer in large particles.
Influence of drop size:
Surface of a globe: 4 p R2
Volume of a globe: 4/3 p R3
Ratio surface / volume: 4 p R2 / 4/3 p R3 = 3 / R
The ratio of the surface compared to the volume is 3 times the reversed order of the radius of the globe, which means that the volume of a drop, or globe, is reduced more quickly than its surface when its radius decreases.
That means that drops of one millimeter of radius will have a ratio surface/volume of 3 cm² per millimeter of suspension, while drops of 0.5 millimeter of radius will have a ratio surface/volume of 6 cm² per ml of suspension.
Increasingly small drops formed from a suspension of minerals, will present an increasingly significant differential of composition between the minerals of the surface and those of their mass, which are the heart of the drops.
The force of the surface tension that is put on the surface of the drop, will deprive it more and more of the mass of its finest minerals, since the volume of the drops decrease more quickly than their surface.
Glazing
Glazing by spraying is the scattering and the suspension of drops in the air in order to direct them towards the surface of the pottery or ceramic to be glazed.
The strong scattering of the suspension helps to better control the application.
The drops pile up on the surface and form a layer
If the pottery or ceramic is very porous, the drops will get harder as it is sprayed and they will settle and form a fairly dry powdery heap.
If the pottery or ceramic is less porous or if the flow of the glaze is affecting the course of events, the drops will have time to form a wet layer, leaving time necessary to the partial reorganization of particles under the effect of surface tension forces.
Glazing Dipping
One dips the porous clay pottery or ceramic to be glazed in a bath of glaze in suspension in water.
The interaction between contacting surfaces of a liquid and a solid forces of the piece make it possible for the water to penetrate into this one, attracting and putting the minerals of the solution on the porous surface.
Dipping is quite a bit different from spraying.
The layer deposited by the interaction between contacting surfaces of a liquid and a solid on the pottery is partly made up of the finest particles attracted onto the surface of the bath by the force of surface tension of the suspension.
These finest and very movable particles wrap around the dipped pottery as it is pushed down into the glaze bath.
They form, just like a clay slip on the plaster of a mold, a super fine casting skin.
When you are dipping, this super fine casting skin is placed between the pottery and the outside glaze coat made up of the particles of the mass, larger in size and denser.
Influence Of The Glazing Method On The Behavior Of The Glaze
The two methods shown on this page lead to different results during firing of the glaze. The forces of surface tension play a big role in the results by allowing a layout and a different selection of the particles put on the piece of pottery.
The particular look of the fired layer and the effects of forces on the motion of a body and of its fusion during firing will be different according to the method used.
The same composition of a glaze, meaning identical minerals in same the proportions, fired in an identical cycle of firing one will be able to observe:
Spraying
Powdered Layer
The powdered layer is made up of glaze granules of different sizes, each one covered by a thin layer of fine particles.
The fine particles melt in the first place and quickly attack the mass of less bulky drops. Small drops melt before the largest and starts a completely different fusion of the glaze, forming fusible points distributed in the mass of the product.
These fusible points develop or progress more quickly than the fusion of slightly larger drops located near their vicinity.
It follows a occurrence of forming a net or network of the layer leading to clusters in large sticky molten drops.
This intermittent layer can be penetrated and allows gases located in the pottery to escape easily.
When the temperature is raised, the forces of surface tension are slowed down, making it possible for clusters to join and form a continuous layer.
This last phase in general makes it possible to obtain glaze healing by carrying out a stage of firing at the highest temperature for a certain length of time.
Wet Layer Of Sprayed Glaze
A wet layer is a tangle of drops whose uninterrupted succession or flow produced in the upper part at the time of wetting makes it possible for the finest particles to form a layer under the action of the forces of surface tension.
Part of the fine particles have time to organize on the surface of the layer and form a smooth and compact skin.
During firing, this compact surface layer will begin to melt before the whole layer and will undergo strong contractions under the effect of shrinking and the high surface tension of the glaze.
The under layers are now at a stage of less advanced softening and the sticking together of this not very uniform compositional mixture will allow ruptures of the layer in fusion leading to cracks and drawing back of the glaze.
This breaking up of the layer of the glaze will allow gases to escape.
With the temperature continuing to rise, the glaze will end up forming a continuous layer and heal over.
Dipping
The layer of the finest particles in direct contact with the pottery will melt first.
The strong elements of a piece of pottery, that are held together will prevent the pulling away of the softened layer and this one will form a relatively continuous and tight envelope.
The gases which will want to escape the pottery will have trouble finding a way through the layer of the glaze and will accumulate.
If the layer of the glaze is thin, small holes will let gases leave more easily.
If the glaze is thick, small holes will be rare and the pressure of the gases will rise up to where it will reach and exceed the bearable limit of the layer of the glaze that is greatly softened by temperature.
Large bubbles will explode on the surface, producing craters of a few millimeters.
Because of this, the pieces produced by dipping will have to be subjected to a close control of the thickness of the glaze, especially if the pottery produces gases during firing.
Detailed attention will have to be paid to the size of the graduals and the grinding of the glaze.
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