|Monthly Tech-Tip |
An technical overview of various glaze type used in the tile industry along with consideration of the materials, processes and firing. By Nilo Tozzi
We can outline an exhaustive set of relationships between the firing and fired characteristics of frits and pigments and their oxide compositions but we cannot do this it for glazes. For glazes, reactions and processes developing during firing never reach a steady state of equilibrium. Thus glazes, after firing, can be considered anything from glasses having a local random distribution of elements (where phases have a more orderly arrangement of elements) to absolutely dispersed crystalline phases.
Consider a simple hypothetical glaze containing 50% frit and 50% natural or synthetic crystalline materials: After firing this glaze will show specific characteristics that we can only partially foresee. We need a more "scientific" approach that will give us a better level of control and the ability to foresee the aesthetic result after firing, we need a lot of information.
Having all of the above information we can begin to visualize the progression of each solid state reaction and the chemical composition of each amorphous and crystalline phase after firing. Still, it is difficult to get a picture close to the real final physical fired product. The number of surprises coming out of the kiln and the number of variables involved quickly make it evident that glazes and their materials are very complicated. Thus I like to imagine a glaze as being like a match: we have to strike a match to see if it works like we have to test a glaze to see its surface.
It is not possible to go into a lot details about workings and kinetics of reactions and transformations because these, in complex glazes, are many and interrelated and can interfere with each other in difficult-to-predict ways.
For instance, there is no use speculating about the transformation of kaolin into mullite if the host glaze is composed of a large percentage of frit and a small amount of kaolin because the latter is just going dissolve into the frit melt before its transformation. However this transformation is important to consider for engobes where a large amount of kaolin and a small amount of frit are present. This is because kaolin dissolves partially and the remaining larger amount contributes to opacity because it transforms into mullite crystals. Another example is barium carbonate: it experiences its decomposition reaction at 1450C (which is higher than similar ones for calcium and magnesium carbonates). But this is irrelevant because in glazes it reacts at lower temperatures with other materials and easily dissolves into glassy matrix (it is impossible to foresee when barium carbonate will start decomposition). A further example is the melting process for feldspars or nepheline: it goes on at lower temperatures because of being accelerated by molten frits or alkaline earth compounds (otherwise feldspar and nepheline contribute to developing a viscous glaze that can impede other reactions). The possible compositions are infinite and it is not practical to attempt to examine all different interactions or lack thereof. We can only outline what happens.
|Transformations in a Single Phase||Processes Between Two Different Phases||Crystallization|
|Water elimination from hydrous materials||Coagulation of melting grains of frits||Separation of liquid phases|
|CO2 elimination from carbonates||Incorporating of crystal phase by molten frits|
|Polymorphic transformations||Reactions among crystalline materials and formation of new phases|
|Structural reorganizations||Partial or total dissolving of crystalline materials in the molten frits|
|New phases forming||Homogenization of molten frits having different composition|
For several processes we are in the presence of solid state reactions where the equilibrium state is slowly reached or never reached and where specific surface area of particles plays a very important role.
In order to simplify the issue I will describe some glazes having specific intrinsic characteristics which have been developed to obtain some aesthetic result.
Engobes have an intermediate composition between bodies and glazes. Their purpose is to hide the color of the ceramic body and eliminate reactions and associated defects caused by the direct contact between glaze and body.
All engobes have 2 primary characteristics (compared to glazes):
The plastic materials in the engobe absorb water in the overlying glaze in a slow and uniform way (without discontinuities in water absorption). Thus quality and quantity of clays in an engobe play a very important role. For single fired tiles we usually have percentages in the 15-30 range (depending on the plasticity of each clay). Together with plastic clays we can also use less plastic kaolins to regulate water absorption and contribute to opacity. Frits, if present, are opaque and feldspars or nepheline are always present in order to impart fusibility using low cost materials.
For instance, consider the following engobe composition as a example of the kind of changes we need for a specific production process.
|Composition Changes for Engobe X|
|+refractory||Partially replace nepheline with quartz and alumina|
|-refractory||Partially replace kaolin with an opaque frit|
|+opaque||Partially replace kaolin with a micronized zircon|
|-opaque||Partially replace kaolin with nepheline|
|+plastic||Partially replace kaolin with clay|
|-plastic||Partially replace clay with kaolin|
Glossy transparent and opaque glazes are prepared from high temperature frits with small additions of kaolin, clay, nepheline, feldspar and zirconium silicate.
First, consider opacity: Opacity can be attained by different mechanisms:
The coating ability of an opaque glaze is expressed by reflectance, expressed as a ratio of the intensity of back-diffused light and intensity of incident light or by percentage of reflectance compared with the reflectance of a standard material.
Variables having a mayor impact on the back-diffused light are:
The following figure displays a reflectance measurement from an opaque red glaze.
A glaze is matt when crystalline materials are dispersed in the glassy matrix. the quantity and dimensions of the crystals for each added crystalline material control the surface texture (varying from semi-glossy to very rough). The resultant irregular surface of the glaze, where crystals are dispersed, causes reflection of incident light in all directions.
To obtain a glassy layer containing dispersed crystals able to modify the surface we can employ the mechanisms of:
To get the best control of dispersed crystal quantities we simultaneously use matt frits and crystalline compounds. Despite this matt glazes are strongly affected by firing cycle, thickness of glaze layer and particle size after milling. Likewise the temperature and the firing cycle are very important because they shape crystallization from frits and dissolving kinetics of added crystals.
During firing we have the formation of an intermediate layer between glaze and engobe, this layer prevents crystallization of frits because it contains a greater amount of aluminum migrating from the engobe (alumina impedes crystallization).
Particle size after milling of different added crystalline materials also affects their quantity and stability because smaller crystals more easily dissolve in the glassy matrix.
Actually, all materials having high melting temperatures are helpful in obtaining matt surfaces. Most used are corundum, alumina, quartz, mullite, wollastonite, rutile, etc. Sometimes, in cases where we want to obtain a rustic surface, we also add materials having large grain dimensions ( e.g. sands of quartz, rutile, zircon and corundum).
Alumina/Corundum: Alumina is a white powder or aggregate of corundum crystals and it can be added to glazes before and after milling. If it is added before milling aggregates are destroyed and the alumina is more reactive (in cases of solid state reactions with other components of the glaze). If it is added after milling it acts like a refractory material, not dissolving into the glass, imparting unique qualities to the surface (depending on its particle size distribution).
|Alumina Oxide - Corundum|
Due to it superior hardness, alumina is also used to improve abrasion resistance of glazes. Small amounts of alumina are also employed to control crystallization, surface texture and chemical resistance, while larger amounts, as already noted, remain partially undissolved and contribute to hardness and opacity (even if its refraction index is not suitable). Of course the glaze becomes matt with a texture depending on the amount of alumina. Excess alumina causes a microscopic porosity of the glaze layer according to the degree to which it becomes more refractory.
Corundum single crystal particles are less brittle and less reactive than alumina and they are employed primarily to produce hard surfaces. However its particle size distribution before and after milling are important because melt fluidity of glazes during firing is strongly affected by its surface area.
Barium carbonate/Calcium carbonate/Magnesium carbonate/Dolomite: All of these compounds decompose in glazes generating reactive oxides that easily dissolve in the melt. If percentages are limited, usually less than 10%, they contribute to fusibility and glossiness. But for higher percentages they contribute to crystal development during cooling and glazes start becoming matt. Of course regarding the development of matt surfaces for these materials, we have to consider the molecular weight of each. For instance, magnesium crystallizes more readily than barium because, in terms of moles, 1% barium equals 4% magnesium.
Clay: The main use of clays is to prepare engobes, usually they are a mixture of illite, kaolin and quartz. The criteria to select a good clay are:
The so called "ball clays" are actually made up of fine particled kaolin and organic matter, both imparting some degree of plasticity.
Kaolin: It is a component of all glazes, primarily for its suspending properties. A good glaze kaolin must be white after firing and plastic. Its transformation to mullite above 1000C is important because, in case it does not dissolve in the glassy matrix, it transforms and makes a small contribution to opacity (having refraction index = 1.64).
Nepheline: The most popular nepheline minerals are Canadian and Norwegian. They are a mixture of albite, microcline (potassium feldspar) and nepheline. In glazes nepheline melts at a lower temperature than feldspars, around 1100C, and contributes to forming a transparent and viscous glassy phase. Additionally, nepheline has a lower iron and titanium content than feldspars.
Quartz: Quartz is the most common structural form of silica. While it undergoes several structural transformations that are of concern for body formulation these have little importance when we use quartz to prepare glazes (because the alfa/beta changes are reversible and the quartz/tridymite transformation is very slow). Quartz dissolves slowly in glazes at a rate proportional to the fluidity of the glassy phase and inversely proportional to it particle size. When quartz dissolves into the melt it imparts more viscosity, if it does not dissolve it partially contributes to opacity and gives a less glossy surface.
|Structural Changes of Silica|
|Alfa Quartz/Beta Quartz||575C|
|Alfa Cristobalite/Beta Cristobalite||220-275C|
|Alfa Tridymite/Beta Tridymite||117C|
Quartz has a specific use when we prepare engobes: we need a high thermal expansion to counterbalance body or glaze behavior. In such cases it is not unusual to add more than 30% quartz (its high expansion before transformation at 575C is particularly beneficial).
Sodium/potassium feldspar: Feldspars have a wide range of compositions but we also find feldspar in other minerals and often with quartz. Pure sodium feldspar (albite) quickly melts at 1150C and produces a glass while potassium feldspar melts at around 1200C producing a more a viscous melt (and a crystalline phase named leucite that contributes to opacity). Sodium feldspars are more suitable for mattes while we prefer potassium feldspar for glossy glazes.
Titanium oxide: Anatase is a not stable phase of titanium at the firing temperatures of glazes, it transforms to rutile or dissolves into the glassy matrix and re-crystallizes during cooling (with rutile structure) or acts as a crystallization catalyst.
Wollastonite: It exhibits a very slight solubility in water but slips containing wollastonite have an alkaline pH of 9-11, this can influence rheological properties even for small percentage additions. During firing, if percentages are less than 10, it easily dissolves in glazes giving more fusibility and more glossiness. For larger amounts it remains partially undissolved or crystallizes again during cooling, contributing to the development of matt surfaces. Likewise, as can be seen from observing fired characteristics, high wollastonite glazes often have a poor hardness and some tendency to craze.
|Thermal Expansion||65 x 10-7 C-1|
|Solubility in Water||95 mg/l|
Zinc oxide: While it has high melting point it easily dissolves in glaze melts giving more fluidity (if less than 10% is present). For higher percentages it can produce crystal phases like Zn2SiO4 (willemite).
The oxygen-zinc bond has a prevalent covalent character and so it is network modifier having a limited effect on thermal expansion. Unfortunately zinc is expensive and glazes containing it are acid leachable. Also, sometimes, for specific zinc oxides and for specific percentages, glaze slips can become tixotropic because during milling the surface of the oxide particles forms a hydroxide layer. For this reason it normal to prepare glazes having matt texture using frits containing high percentages of zinc oxide.
Zirconium silicate: Its decomposition in glass matrix is slight and slow. It strongly contributes to opacity because its refractive index is high (particularly when we use micronized zircon, having a particle size less than 5 microns). Zircon sand is used for rustic glazes and zircon flour for semi- transparent glazes. Its specific gravity is high and slips or glazes containing zircon thus have high density.
|Melting Temperature||Above 1600C decomposes to zirconium and silicon oxides|
|Thermal Expansion||72 x 10-7 C-1|
Actually automatic equipment to prepare printing pastes are not a recent innovation, they are in in every day use. Tiles are characterized by decoration and there are a large number of printing machines in glazing lines, however management of inks is often difficult. A Tintometric system is able to prepare a colored ink on the basis of a colorimetric measurement in the lab and using a limited number of pigments. The main characteristics of tintometric systems are:
The application of colors by the inkjet system, without contact between printer an tile and with high resolution, opens many new prospects for tiles decoration. Ink jet technology is of course widely used for paper printers but it is quite new and still under development for tiles. While the cost of inks and equipment are high the method guarantees great flexibility (much of the potential is still to be explored.
The new machines use inks quite similar to those traditionally used for glaze line printing machines. Some are using a four-color process while others work using all available pigments (thus without color limitations) and also using dry powders.
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By Nilo Tozzi