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Surface Tension and Glazing


  • by Smart.Conseil

    Translated from French by Edouard Bastarache.

    Gibbs’ Theorem:
    According to the principles of thermodynamics, the elements which make up the surface layer of a solution and those of its mass are different.

    Surface Tension: 
    It is a force resulting from cohesiveness, which lowers to a minimum the number of molecules on the surface of a liquid. This creates a kind of invisible enveloppe which occupies the smallest possible surface. The surface tension represents the force of the film on the liquid surface.
    When one applies this principle to a minerals suspension, one can observe 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 spontaneously.

    Application to an isolated drop of the suspension:
    A drop is a volume of insulated liquid whose cohesion is ensured by the forces of surface tension. In the absence of external influence, the drops have a spherical form.

    Thus if one applies this reasoning to a drop of this suspension, the finest particles come to cover the surface of this one and leave in the centre a mass of a suspension richer in large particles.

    Influence of drop size:
    Surface of a sphere: 4 p R2
    Volume of a sphere: 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 inverse of the radius of the sphere, which means that the volume of a drop (sphere) is reduced more quickly than its surface when its radius decreases.
    Ex: drops of 1 mm of radius will have a ratio surface/volume of 3 cm² per ml of suspension while drops of 0.5 mm of radius will have a ratio surface/volume of 6 cm² per ml of suspension.

    Thus, for increasingly small drops formed from a suspension of minerals, these will present an increasingly significant differential of composition between the minerals of the surface and those of their mass (heart of the drops). The force of the surface tension exerting on the surface will impoverish more and more the mass of its finest minerals, since the volume of the drops decrease more quickly than their surface.

    Glazing 

    Glazing by spraying:
    The principle is to disperse a suspension of drops in the air in order to direct those towards the surface of the ware to be glazed. The strong dispersion of the suspension helping with better controlling the application.

    The drops pile up on the surface and form a layer

    • If the shard is very porous, the drops will be solidified progressively as they settle and will form a powdery heap.
    • If the shard is less porous or if the flow of pulverization is too important 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 by dipping:
    One dips the porous shard to be glazed in a bath of glaze in suspension in water. The capillarity forces of the shard make it possible for the water to penetrate into this one, attracting and thus plating minerals of the solution on the porous surface.

    The case of dipping differs much from spraying. The layer deposited by capillarity on the shard 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 mobile particles wrap the dipped ware as it progressively penetrates the glaze bath.

    They form, just like a clay slip on the plaster of a mould, a ” super fine casting skin “. But in the case of dipping this ” super fine casting skin ” is inserted between the shard and the external 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 quoted on this page lead to different results during firing of the glaze. The forces of surface tension play a dominating role in the results by allowing a layout and a different selection of the particles deposited on the shard. The aspect of the fired layer and the dynamics of its fusion during firing will be different according to the method used.
    Thus for the same composition of a glaze (identical minerals in same the proportions) fired in an identical cycle of firing one will be able to observe:

    1) Spraying:

    a) 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. Thus small drops melt before the largest and start the heterogeneous fusion of the glaze, forming fusible points distributed in all the mass of the product.
    These fusible points accelerate the fusion of slightly larger drops located near their vicinity and so on. It follows a phenomenon of reticulation of the layer leading to clusters in large viscous molten drops. This discontinuous layer is permeable and allows gases located in the shard to escape easily.
    With temperature rising, the forces of surface tension are slowly slackened, making it possible to clusters to join and form a continuous layer. This last phase in general makes it possible to obtain glaze healing over by carrying out a stage of firing at the highest temperature during a certain time.


    b) Wet layer of sprayed glaze:

    The wet layer is a tangle of drops whose continuity produced in the upper part at the time of wetting made it possible that the finest particles form a layer under the action of the forces of surface tension. Thus part of the fine particles had the 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 of the layer and will undergo strong contractions under the effect of shrinking and the high surface tension of the glaze in formation. The under-layers being at a stage of less advanced softening, the cohesion of this not very homogeneous mixture will allow ruptures of the layer in fusion leading to cracks and retractions.
    These discontinuities of the layer of the glaze will allow gases to escape. Then the temperature continuing to rise, the whole will end up forming a continuous layer and heal over.

    2) Dipping: 

    The layer of the finest particles in direct contact with the shard will melt in first place. The strong cohesion of the shard will prevent the retraction of the softened layer and this one will form a relatively continuous and tight envelope. The gases which will want to escape the shard will have trouble to find a way through the layer of the glaze and will accumulate.
    If the layer of the glaze is thin, discontinuities (small holes) will let gases leave more erasily.
    If it is thick, discontinuities will be rare and the pressure of the gases will rise up to reach and exceed the bearable limit of the layer of the glaze strongly softened by temperature. Large bubbles will explode on the surface, producing craters of a few millimetres.

    For this reason, the pieces produced by dipping will have to be subjected to a rigorous control of the thicknes of the glaze, especially if the shard produces gases during firing. A very detailed attention will have to be paid to the granulometry and the grinding of the glaze.

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