Glaze Defects
Encyclopedia
Glaze defects are any flaws in the surface quality of a ceramic glaze
, its physical structure or its interaction with the body.
pattern of cracks penetrating the glaze. It is caused by tensile stresses greater than the glaze is able to withstand. Common reasons for such stresses are: a mismatch between the thermal expansion
s of glaze and body; from moisture expansion of the body; and in the case of glazed tile
s fixed to a wall, movement of the wall or of the bonding material used to fix the tile to the wall.
The cracks can allow the ingress of water
, dirt
and bacteria
into the cracks. Once fired ware tends to be more resistant to crazing due to better development of the glaze/body interfacial layer, which reduces stress gradients between the glaze and body.
The causes of crazing include:.
Thermal expansion mis-match between the glaze and the body. Poor glaze/body fit is the main cause of crazing and can be due to:
Moisture expansion of the body - porous bodies will swell slightly due to absorption of moisture. Where glazes are in only slight compression this can be sufficient to bring them into tension. The problem results in delayed or secondary crazing which occurs over a period of time after the ware has been produced.
Glazing too thickly. This is a common cause of crazing. Glazes, which should be craze resistant, can craze if applied too thickly. This is because the further the glaze surface is away from the body the lower the compression acting on it.
Thermal shock. Opening the kiln too soon above 100°C can cause crazing and dunting. Above 200°C catastrophic failure can occur due to the volume changes at the cristobalite inversion (around 225°C)
Glaze Fit.
The matching of the thermal expansion of a glaze to that of the body on which it is held. To prevent crazing the glaze must be in compression when the ware has been cooled from the kiln to room temperature; to achieve this, the thermal expansion of the glaze must be less than that of the body.
Steger's Crazing Test is a method for the assessment of the glaze fit. It is undertaken by measuring any deformation on cooling of a thin bar that was glazed only on one side. A common method of testing glazed ceramic ware for crazing resistance is to expose pieces of ware to the steam in an autoclave at a minimum of 50 p.s.i..
Seger's Rules are a series of empirical rules put forward by the H.A.Seger for the prevention of crazing and peeling. To prevent crazing, the body should be adjusted as follows: decrease the clay, increase the free silica; replace some of the ball clay
by kaolin; decrease the feldspar
; grind the silica more finely; biscuit
fire at higher temperature. Alternatively, the glaze can be adjusted: increase silica and/or decrease fluxes; replace some SiO2 by B2O3; replace fluxes of high equivalent weight by fluxes of lower equivalent weight. To prevent peeling, the body or glaze should be adjusted in the reverse direction.
and cadmium
are the metals of greatest concern, although testing can be extended to include others. The propensity for any glaze to release metal may depend on complex interactions between the formulation used, any applied decoration and the kiln atmosphere.
Monitoring the level of metal release from glazed ware forms part of the Quality control
procedures of all reputable producers. Test methods are specified according national and international standards, although testing usually involves: the ware being immersed or filled with a 4% acetic acid
solution; covered and left for 24 hours at room temperature, although if cooking ware is being tested higher temperatures are needed; the acetic acid solution decanted from the ware and the concentration of leached metal measured by Atomic absorption spectroscopy
. Acceptance limits are enforced by legislation, and whilst varying between countries all are within the ppm
range. Some of the most well recognised legislation are: across Europe 'EC Directive 84/500/EEC 1984'; for the UK 'GB Ceramic Ware (Safety) Regulations SI 1647, 1988'; and for the USA 'FDA Compliance Policy Guide 7117.06 and 7117.07 for cadmium and lead.'
that burst but was only partially healed. The bubbles are most often from gas
that originates from air trapped between the particles of powdered glaze as the glaze begins to mature, or from gases evolved from carbonate
compounds.
A specific example of pin-holes is Spit-out. These are pin-holes or craters sometimes occurring in glazed non-vitreous ceramics while they are in the decorating kiln
. The cause of this defect is the evolution of water vapour, adsorbed by the porous body, during the period between the glost firing and the decorating firing, via minute cracks in the glaze..
Ceramic glaze
Glaze is a layer or coating of a vitreous substance which has been fired to fuse to a ceramic object to color, decorate, strengthen or waterproof it.-Use:...
, its physical structure or its interaction with the body.
Body/glaze interaction problems
Glaze defects can be as a result of the incompatibility of the body and the selected glaze, examples including Crazing and Peeling.Crazing
Crazing is a spider webSpider web
A spider web, spiderweb, spider's web or cobweb is a device built by a spider out of proteinaceous spider silk extruded from its spinnerets....
pattern of cracks penetrating the glaze. It is caused by tensile stresses greater than the glaze is able to withstand. Common reasons for such stresses are: a mismatch between the thermal expansion
Thermal expansion
Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...
s of glaze and body; from moisture expansion of the body; and in the case of glazed tile
Tile
A tile is a manufactured piece of hard-wearing material such as ceramic, stone, metal, or even glass. Tiles are generally used for covering roofs, floors, walls, showers, or other objects such as tabletops...
s fixed to a wall, movement of the wall or of the bonding material used to fix the tile to the wall.
The cracks can allow the ingress of water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
, dirt
Dirt
Dirt is unclean matter, especially when in contact with a person's clothes, skin or possessions when they are said to become dirty. Common types of dirt include:* dust — a general powder of organic or mineral matter...
and bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
into the cracks. Once fired ware tends to be more resistant to crazing due to better development of the glaze/body interfacial layer, which reduces stress gradients between the glaze and body.
The causes of crazing include:.
Thermal expansion mis-match between the glaze and the body. Poor glaze/body fit is the main cause of crazing and can be due to:
- Under-firing (earthenware body types) resulting in failure to develop sufficient body thermal expansion.
- Firing too quickly, resulting in failure to achieve sufficient heatworkHeatworkHeatwork is the combined effect of temperature and time. It is important to several industries:*Ceramics*Glass and metal annealing*Metal heat treating...
. - Low thermal expansion body.
- High thermal expansion glaze.
- Over-firing of vitreous ware.
Moisture expansion of the body - porous bodies will swell slightly due to absorption of moisture. Where glazes are in only slight compression this can be sufficient to bring them into tension. The problem results in delayed or secondary crazing which occurs over a period of time after the ware has been produced.
Glazing too thickly. This is a common cause of crazing. Glazes, which should be craze resistant, can craze if applied too thickly. This is because the further the glaze surface is away from the body the lower the compression acting on it.
Thermal shock. Opening the kiln too soon above 100°C can cause crazing and dunting. Above 200°C catastrophic failure can occur due to the volume changes at the cristobalite inversion (around 225°C)
Glaze Fit.
The matching of the thermal expansion of a glaze to that of the body on which it is held. To prevent crazing the glaze must be in compression when the ware has been cooled from the kiln to room temperature; to achieve this, the thermal expansion of the glaze must be less than that of the body.
Steger's Crazing Test is a method for the assessment of the glaze fit. It is undertaken by measuring any deformation on cooling of a thin bar that was glazed only on one side. A common method of testing glazed ceramic ware for crazing resistance is to expose pieces of ware to the steam in an autoclave at a minimum of 50 p.s.i..
Seger's Rules are a series of empirical rules put forward by the H.A.Seger for the prevention of crazing and peeling. To prevent crazing, the body should be adjusted as follows: decrease the clay, increase the free silica; replace some of the ball clay
Ball clay
Ball clays are kaolinitic sedimentary clays, that commonly consist of 20-80% kaolinite, 10-25% mica, 6-65% quartz. Localized seams in the same deposit have variations in composition, including the quantity of the major minerals, accessory minerals and carbonaceous materials such as lignite...
by kaolin; decrease the feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
; grind the silica more finely; biscuit
Bisque (pottery)
Bisque porcelain is unglazed, white ceramic ware Examples include bisque dolls.Bisque also refers to "pottery that has been fired but not yet glazed...
fire at higher temperature. Alternatively, the glaze can be adjusted: increase silica and/or decrease fluxes; replace some SiO2 by B2O3; replace fluxes of high equivalent weight by fluxes of lower equivalent weight. To prevent peeling, the body or glaze should be adjusted in the reverse direction.
Peeling
The breaking away of glaze from ceramic ware in consequence of too high a compression in the glaze layer; this is caused by the glaze being of such a composition that its expansion coefficient is too low to match that of the body. It is the opposite of crazing, as are the preventative steps: see Seger's Rule above. Peeling is also known as shivering.Metal release
Regulations have existed since the late 1960s to protect consumers from the potential risk of toxic materials, mainly metals, being released from glazes into drink and foodstuffs. LeadLead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
and cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
are the metals of greatest concern, although testing can be extended to include others. The propensity for any glaze to release metal may depend on complex interactions between the formulation used, any applied decoration and the kiln atmosphere.
Monitoring the level of metal release from glazed ware forms part of the Quality control
Quality control
Quality control, or QC for short, is a process by which entities review the quality of all factors involved in production. This approach places an emphasis on three aspects:...
procedures of all reputable producers. Test methods are specified according national and international standards, although testing usually involves: the ware being immersed or filled with a 4% acetic acid
Acetic acid
Acetic acid is an organic compound with the chemical formula CH3CO2H . It is a colourless liquid that when undiluted is also called glacial acetic acid. Acetic acid is the main component of vinegar , and has a distinctive sour taste and pungent smell...
solution; covered and left for 24 hours at room temperature, although if cooking ware is being tested higher temperatures are needed; the acetic acid solution decanted from the ware and the concentration of leached metal measured by Atomic absorption spectroscopy
Atomic absorption spectroscopy
Atomic absorption spectroscopy is a spectroanalytical procedure for the qualitative and quantitative determination of chemical elements employing the absorption of optical radiation by free atoms in the gaseous state. In analytical chemistry the technique is used for determining the concentration...
. Acceptance limits are enforced by legislation, and whilst varying between countries all are within the ppm
Parts-per notation
In science and engineering, the parts-per notation is a set of pseudo units to describe small values of miscellaneous dimensionless quantities, e.g. mole fraction or mass fraction. Since these fractions are quantity-per-quantity measures, they are pure numbers with no associated units of measurement...
range. Some of the most well recognised legislation are: across Europe 'EC Directive 84/500/EEC 1984'; for the UK 'GB Ceramic Ware (Safety) Regulations SI 1647, 1988'; and for the USA 'FDA Compliance Policy Guide 7117.06 and 7117.07 for cadmium and lead.'
Blisters
A large bubble sometimes present as a fault in ceramic ware. Blisters appear as large bubbles either just below or penetrating the surface, leaving sharp, rough edges which collect dirt. The surface of the glaze is very unpleasant and looks like a boiled mass of bubbles, craters and pinholes.Crawling
A defect that appears as irregular, bare patches of fired body showing through the glaze where it has failed to adhere to or wet the body on firing. The cause is a weak bond between glaze and body; this may result from greasy patches or dust on the surface of the bisque ware or from shrinkage of the applied glaze slip during drying. The fault is more likely to occur with once-fired ware such as sanitaryware..Metal Marking
Metal marks are dark lines, often accompanied by damage in the glaze, caused by the deposition of metal during the use of metal utensils. The cutlery, or other relatively soft metal, will leave a very thin smear of metal on pottery ware if the glaze is minutely pitted. A glaze may have this defective surface as it leaves the glost kiln, or it may subsequently develop such a surface as a result of inadequate chemical durability. The fault is also known as cutlery marking.Pin-hole
A fault that is commonly the result of a bubble in the glaze when it was moltenMölten
Mölten is a comune in South Tyrol in the Italian region Trentino-Alto Adige/Südtirol, located about 60 km north of Trento and about 12 km northwest of Bolzano .-Geography:...
that burst but was only partially healed. The bubbles are most often from gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
that originates from air trapped between the particles of powdered glaze as the glaze begins to mature, or from gases evolved from carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....
compounds.
A specific example of pin-holes is Spit-out. These are pin-holes or craters sometimes occurring in glazed non-vitreous ceramics while they are in the decorating kiln
Kiln
A kiln is a thermally insulated chamber, or oven, in which a controlled temperature regime is produced. Uses include the hardening, burning or drying of materials...
. The cause of this defect is the evolution of water vapour, adsorbed by the porous body, during the period between the glost firing and the decorating firing, via minute cracks in the glaze..