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What Causes Dimple/craters On The Tile Surface?


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hello everyone,

 

I'm like a baby in tiles industry, just joined the company for around 6 months, I would like to ask you guys here what actually causes dimple/craters defect on tile surface. The problem is, the number of the dimple/craters are just one or two per tile, and it is not continuous. I have run a test by blowing contaminate dust on tile surface before the glaze get dried just after application, there is a dimple however it is not the same as we had in the production. I used to think this defect caused by an oil, yet I still couldn't recreate the same dimple as we faced now. could you guys please help which area should I check to solve this?

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Abil:

 

A close up picture would help. Your definition of "dimple/crater" please?  My definition of a crater is a recessed area greater than 1/8" Under that size I would classify as a pin hole. Tile business would mean a tunnel kiln with continuous feed, an automated fountain glazer, and a very large series of hydraulic tile presses. Which would also mean a talc body perhaps, or veneering porcelain over a talc body. Talc body would mean high magnesium content- one possibility. A food safe glaze I would suspect, less likely to be that. Could be as simple as an extra ten minutes at peak maturity temp until it was done off-gassing. Hard to tell without info.

 

Nerd

 

Edit: I will go with door #3 on the limited info you gave: just slightly slightly over fluxed glaze.Run some test tiles: a series with 2% less flux, and a second series with 4% less flux. Might just be right at the border, slightly over: have you ran a formula limits analysis of your glaze? Is the sodium, potassium levels at the peak of limits? Boron?

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The technical answer is that there must be "something" trapped under the glaze layer after the glaze melts enough to provide a complete liquid phase interconnecting the particles of the glaze layer. This "something" must then disappear in a way that leaves a depleted spot in the final glaze layer, aka dimple.  The "something" can be air trapped in a pore of the immature clay body, or it could be "something" that generates gas as it reacts to become a part of the glaze, or it could be something that "burns out" from the surface as the glaze matures. 

Further, the glaze must be viscous enough to not flow into the space occupied by the "something" as the "something" disappears.  Surface tension of the melt can also retard the leveling of the dimple, especially if the bottom of the dimple is bare of glaze.  Viscosity and surface tension are sensitive to temperature, both having a somewhat steep decrease with temperature. 

There are lots of opinions as to the identity of the "something" but there is little explicit data to generalize the opinions to reliable fixes. The best we have are fixes that worked for someone once upon a time.

 

Some probable important variables are:  dust on the surface be fore glazing, oil on the surface, impurities in the glaze batch, carbonates in the glaze batch or within the clay body that have delayed decomposition, firing rates, slowly combusting debris on the surface of the glaze, degree of reduction at a critical time, rate of temperature rise, hold times, and .... .  The severity may be the result of combinations of variables rather than a single variable. 

 

If you cross section the tiles through a "dimple" and study the interior structure and composition of the exposed material, you may gain additional insight to what is happening.  

 

Search the archives of the Journal of American Ceramics Society, and its British and European counterparts, for outgassing of ceramic tile. 

 

LT

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