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Calcine, Calcination
The calcining process is used to remove some or all unwanted volatiles from a material (e.g. H2O, CO2) and/or to convert a material into a more stable or durable state. Varying temperatures are required to calcine various materials. For example, kaolins are calcined to form molochite. Normally materials are ground after calcining. Often calcining can produce a less stable from of a material that gradually wants to revert to the former carbonated or hydrated state. For a good example of this, mix calcium carbonate with kaolin and make a bar and fire it. Out of the kiln it will appear to be a hard ceramic but after several days it will absorb CO2 from the air and completely fracture into a powder. Pour water on it and it will immediately fracture and generate an amazing amount of heat.
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- (Materials)
Bone Ash - Ca5(OH)(PO4)3
Calcium Phosphate
Pictures
The top bar is a mix of calcium carbonate and clay fired to cone 6. The bottom is a couple of minutes after water was poured onto it.
Calculated Thermal Expansion
INSIGHT software calculates the thermal expansion of a glaze and part of its chemistry. The number it reports is based on the contributing expansion factors and amounts of each oxide in the formula. Results are determined by the set of expansion numbers and method of additive calculation method chosen (based on formula or mole%). Thermal expansion values predicted by calculation are relative (not absolute) and apply within 'systems'. Thus, if a glaze calculates to a higher expansion than another, and is in the same system, then it is more likely to craze. Conversely, so solve a crazing problem, move the expansion number downward.
Another factor is the homogeneity of the material. Frits, for example, compared to raw materials, have glass particles of the same chemistry, thus every particle is going to do something predicable during melting. Raw materials, on the other hand, have particles of possibly a dozen different minerals, each having it's own complex melting behavior that is a product of it's mineralogy as well as it's chemistry and particle size and shape. In addition, these particles interact in complex ways. Thus, the calculated thermal expansion of some materials may not be accurate.
The thermal expansion of bodies cannot be calculated either because there are too many factors other than chemistry that impact the thermal expansion of a clay body. The most fundamental in that clay bodies do not melt like glazes, the oxides do not form a glass; thermal expansion calculations depend completely on this assumption. Expansion calculations also depend on the assumption that no crystallization is taking place (once a glass crystallizes is thermal expansion will change completely, clay bodies are loaded with crystallization). Particle size distribution, mineralogy of the particles, the degree to which the body is fired are other factors that affect expansion. For example, while the SiO2 content may be similar in two bodies of similar chemistry one may have most of the SiO2 in quartz grains and the other might have it in feldspar and kaolin. These will have vastly different thermal expansions.
Real-world expansion numbers are extremely small and refer to the amount by which an item expands per degree rise in temperature. INSIGHT removes the decimal to produce a simple number that generally falls between 5 and 8 (for the default expansion number set). This number is relative only. Thus if a glaze is crazing you need to adjust the formula to bring the expansion number down. If it is shivering you do the opposite. The amount by which you change it comes with the experience of seeing a fired result and comparing it with the degree of change in the chemistry. Different glazes systems behave differently in the degree to which they respond to chemistry changes. For example, if a glaze is crazing badly and the calculated thermal expansion is 8.0, then try effecting a chemistry change that will bring that number down to 7.5, test and see whether it needs to be reduced further or less.
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- (Glossary)
Glaze Compression
Every solid has a thermal expansion, that is, an a... - (Glossary)
SiB:Al Ratio
This number is reported by INSIGHT software as par... - (Glossary)
Si:Al Ratio
This number is reported by INSIGHT software as par...
Candling
The practice of slow firing ware through the critical temperature surrounding the boiling point of water. This is done in situations where a drier is not available, it prevents cracking and explosions associated with steam trying to vent out of ware that is not completely dry. The situation is aggravated when ware has a thick cross section. Kilns are often candled overnight on very low heat and then the firing is continued in the morning.
Carbon trap glazes
Glazes with variegated patterns of grey and black from carbon trapped below the surface.
The effect is created by fuel firing without adequate oxygen in early stages to build up soot (carbon) on the surface of ware. As the firing continues, the carbon trap glaze begins to melt before the carbon sitting on the surface burns away. Carbon is a refractory material and will stay in a glaze as long as there is no oxygen to combine with it. Typically this type of glaze includes soda ash or other soluble alkaline fluxes which will migrate to the surface of the raw glaze as it dries, forming a crust of alkalis which will melt earlier than the rest of the glaze, thus facilitating the carbon trapping.
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CAS Numbers
CAS (Chemical Abstracts Service) is a registry of chemical identification numbers maintained by the American Chemical Society. Although ceramic materials are generally minerals or processed minerals rather than chemicals, most do have numbers in the system. However many of the numbers do not have a high level of specificity (e.g. there is one number for kaolins, one for feldspars, one for clays). CAS numbers make it easier and faster to search online databases since the name of a given material can have many forms (sometimes dozens). Since regulatory bodies in many countries require that companies and individuals keep good records of the nature and hazards of the materials they have on hand, reference to these numbers is valuable.
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Casting, Slip Casting
Forming pottery by pouring deflocculated (water reduced) clay slurry into plaster molds. In the process the absorbent plaster pulls water from the slurry and over a period of minutes a layer builds up against the mold surface. The slurry is then poured out and within a short time the item shrinks slightly and can be removed from the mold.
In the hobby industry, 'ceramics' is seen as separate from 'pottery' and is viewed as more of a craft than an art (although in recent years much more casting of fine porcelain is being done on a small scale). The term 'ceramics' has come to refer to casting of low temperature terra cotta or talc slurries. In industry where production is key, casting is used in many more ways (using high-tech processes, equipment and highly tuned formulations).
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Pictures
A slip cast bowl just removed from its plastic mold
Scale, calipers and fired test bars to be measured for shrinkage
Celadon Glaze
A green or blue-green reduction high or medium temperature glaze that has been stained using iron oxide. Celadons were developed by the ancient Chinese and there are many books on the subject. Typically celadon glazes are employed on porcelain but can also be used effectively on stonewares. Often celadons possess their high gloss because of high amounts of sodium and potassium, these oxides also cause the characteristic crazing often seen. However this problem can be solved by substituting some of the Na2O with MgO and increasing the SiO2 (using ceramic chemistry calculations of course).
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- (Recipes)
77E06B - Plainsman Celadon Cone 10R
Standard celadon used at Plainsman Clays for many years
2008-07-22 - Just adjust the amount of iron to get the shade of...
Pictures
Celadon cone 10R glaze (about 3.5% iron oxide) with G1947U transparent liner glaze
Crazing in cone 10 reduction celadon glazes is common because they are high in K2O/Na2O
This one inch tall mug fired at cone 5 with Alberta Slip+20% frit 3134 is similar to a cone 10 reduction celadon glaze.
Close-up of bubbles (upper half) that become evident in a double thickness of a cone 10 celadon glaze that is lacking in flux
Ceramic
A man-made solid produced by the fusion of mineral substances in a kiln.
The term 'ceramic industry' or 'pottery industry' are subjective terms that can mean different things in different circles. In recent years the field of non-oxide ceramics has become popular, thus the term 'ceramics' now generally refers to thermally treated, non-metal, non-gaseous products like glass, sanitary ware, spark plugs, porcelain, abrasives, etc.
Ceramic Chemistry
There is a close link between the way glazes fire in a kiln and their chemistry. Physical properties like color, hardness, melting temperature, thermal expansion, leachability, etc are all direct products of the chemistry. Understanding the relationship between the absolute and relative amounts of the common oxides appearing in a typical glaze formula (not a recipe) to the physical presence of a fired glaze itself is the key to control. An education in glaze chemistry usually starts with a study of each of the dozen or so oxides that a fired glaze is conceptually structured from and what properties each contributes; then a study of what materials contribute what oxides and finally how to use a computer program to study existing glazes and explain their behavior and start adjusting them to alter properties or fix problems. Understanding this is well within the reach of anyone and is actually much simpler than trying to grasp the relationship between the recipe of a glaze and its behavior.
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Ceramic Material
At first it might seem strange to define this, but it is not as obvious as it seems. In ceramics the concept of a material is different for different people. To a purchasing agent it is a commodity. To a geologist it is a mineral or mix of minerals. To a mining crew it is a stockpile of rocks or clay. To a processing department it presents a set of challenges to grind, separate, purify, size and package. To a production department it is a powder that makes up part of the recipe of a glaze or clay body. To a lab technician materials have a physical presence that can be tested and the results of these tests can be expressed on a data sheet. Labs can also deduce the chemistry of a material. But to a glaze chemist, a material is also a warehouse of oxides. He/she assesses the materials physical and chemical contribution to determine how to use it in a formulation. Recipes thus become 'material independent', the oxide needs can be supplied from whatever materials are at hand.
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Chromaticity
A method for evaluating color in the ceramic tile industry.
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Clay
Clays occur when parent clay-making rocks (there are many e.g. feldspars) break down physically and hydrate to form new mineral particles with new properties. This hydration involves insertion of complete water molecules into the crystal structure (normally with most minerals oxides are converted to hydroxides on hydration).
Clays have plasticity. This property is a product of the fact that individual clay particles have surface chemistry that give them an affinity for water and electrolytic charges that give them an affinity for each other. The water thus becomes a lubricant and glue that gives billions of particles the opportunity to express the property of plasticity. From a mineral point of view, clays are hydrous-layer silicates of aluminum (kaolin is pure clay mineral, its chemistry is Al2O3.2SiO2). They are created by the weathering and altering of parent minerals (in this context referred to as 'clay-making minerals'). Clays have a wide range of particle sizes and shapes and mineralogies and these are related to the identity of parent rocks, mode of conversion and whether they are primary (on site of alteration) or secondary (moved by water or wind). These factors produce different plasticities and drying shrinkages. The different chemistries of clays and amounts of contaminents produce different firing behaviors (e.g. temperature of vitrification, color, strength, efflorescence).
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Clay Shrinkage
All clays shrink during drying. Generally the amount of drying shrinkage is related to plasticity, the more plastic a clay the more it dries (and likely the more prone it will be to drying cracks). Drying shrinkage is also related to particle size, the smaller the ultimate particle size the more the shrinkage. Drying shrinkage determination on a clay is an easy test that can enable you to effectively compare it with other clay materials (for plasticity and particle size). Of course, to get consistent results on the same material requires the same water content. Some clays shrink so much and dry so slowly that it is not practical to make bars of the pure material (e.g. bentonite, ball clay). In these cases silica is mixed with the specimen (e.g. 50:50) or it is blending with a calcined version of itself.
Fired shrinkage (shrinkage from dry to fired) is an indicator of degree of vitrification. As a clay is fired higher it shrinks more and more until a point of maximum shrinkage (after which swelling occurs as a precursor to melting). If fired shrinkages are measured over a range of temperatures for a body it is possible to create a graph to get a visual representation of the body's maturing range. The shrinkage plotted against temperature produces a line that increases to a maximum, levels out then drops off.
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- (Glossary)
Ultimate Particles
Physical particles of materials are those we can m... - (Articles)
The Physics of Clay Bodies Learn to test your clay bodies and recording the results in an organized way and understanding the p... - (Glossary)
Porosity
In ceramic testing this term generally refers to t...
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Coatings, ceramic coatings,
rfc
Refractory ceramic coatings are sprayed onto the insides of kilns and on elements to achieve several goals. Coated elements are claimed to last up to 10 times longer. Coated kilns are more efficient and last longer. Coating technology has been developed into successful commercial products by Feriz Delkic ( pronounced Ferris) of International Technical Ceramics, Inc. Although ceramic coatings are expensive, they do save money in the long run.
COE, Co-efficient of Thermal Expansion
A measure of the reversible volume or length change of a ceramic material with temperature. The more it expands during heating the more it contracts while cooling down. Glazes that do not have a similar thermal expansion to the body cause problems like crazing, shivering, and weakened ware. At 2000F fused silica (non crystalline) has an expansion of almost zero (compared to room temperature) whereas quartz mineral has the highest expansion 1.5%. Fused alumina is 0.9% and stabilized zircon 0.8%.
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- (Glossary)
Cristobalite
A crystalline form of silica (quartz is also) avai... - (Tests)
COLE - Co-efficient of Linear Expansion
- (Glossary)
Corning Ware, Pyroceramics, Pyrex
While this is a trade-name of a specific type of c... - (Project)
Ceramic Properties
A property in this context is a created physical p... - (Properties)
Body Thermal Expansion
Materials in this class are added to bodies to red... - (Tests)
CIGF - Boiling Water:Ice Water Glaze Fit Test
- (Troubles)
Glaze Crazing and Shivering
Questions to ask and ways to analyse crazing probl... - (Glossary)
Ovenware
Ovenware clay bodies have a lower thermal expansio... - (Glossary)
Oxide System
In ceramic glaze calculation, a 'system' refers to... - (Glossary)
Calculated Thermal Expansion
INSIGHT software calculates the thermal expansion ... - (Glossary)
Glaze Compression
Every solid has a thermal expansion, that is, an a... - (Glossary)
Cordierite Ceramics
Cordierite ceramics get their properties from the ...
Colloid
Colloidal particles are so small and light that they do not settle in water. The movement of water molecules is enough to keep them in suspension. It is important to remember that colloidal particles occur in a suspension, not a solution (if a beam of light is visible through a liquid it is likely a solution, although it could be a suspension of low specific gravity). Bentonite contains colloidal particles (which also carry an electrolytic charge). Materials can be ground to nano-sized colloidal particles in a ball mill, for example.
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Pictures
This 1000 ml 24 hour sedimentation test compares Plainsman A2 ball clay ground to 10 mesh (left) with one that same material ball milled (right). There is no sediment in the milled material.
Colorant
A material that transforms a glossy or white glaze into a colored glaze. Colorants can be raw metal oxides (e.g. iron oxide, chrome oxide) or smelted (e.g. stains). Potters and smaller companies often use raw colorants whereas industry employs stains. Unlike stains which are prefired, the color of a raw powder colorant likely bears no resemblance to the color it will produce in a glaze. In ceramics, color is a matter of chemistry. The color produced depends on the chemistry of the host glaze and of the mix of colorants added. The same metal oxide can participate in many color systems. Some colorants produce the same color across a wide range of host glazes (cobalt), others are very sensitive to the presence or absence of specific helper or hostile oxides (chrome-tin). Colors are the most vibrant in transparent glazes where there is depth. In opaque glazes colorants tend to produce pastel shades. Some colors are potent, 1% can produce a strong color. Others are weak and 10% make be needed.
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Pictures
Example of three different colors of iron oxide pigments
Alberta Slip GA6C recipe on right (normal) and on left where a Boraq has been used as the flux instead of Ferro Frit 3134. The MgO is destroying the effect!
Cone
A pyramid-shaped ceramic device used to quantify the amount of heat delivered by a kiln. These devices are formulated from different mineral mixtures and numbered accordingly. They are placed in a kiln so they can be viewed during firing and when a cone begins to bend it is closely monitored and the firing is terminated when it reaches a specific position.
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Cone plaque
A stand or rest for hold cones during firing. Plaques are important to assume that cones are placed at a consistent depth and angle firing after firing.
Pictures
Cone plaques and cones from a cone 10R firing at Plainsman Clays.
Controller
An electronic device attached to a kiln. Controllers are usually capable of firing a kiln to a specific schedule and can shut it off at the right time, soak it for a specified period, and cool it down at a controlled rate.
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Cordierite Ceramics
Cordierite ceramics get their properties from the presence of cordierite crystals. They are made using high purity talc with low CaO content that will seed the development of the crystals. The cordierite crystals are grown and aligned in the ceramic matrix by the use of firing curves that apply the exact temperature and cooling rate needed. The low thermal expansion material that this process creates has great resistance to failure induced by sudden temperature change. Automotive catalytic converters constantly heat up and cool down, they are made from cordierite bodies. Cordierite is also refractory. Cordierite kiln shelves are common. While cordierite ceramic vessels could be made, the material has such a low thermal expansion it is very difficult (or impossible) to match a glaze (without crazing).
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Corning Ware, Pyroceramics, Pyrex
While this is a trade-name of a specific type of ceramic available, the term is also used in a generic sense to refer to porcelain tableware that has an extremely low thermal expansion. While a trade secret of the Corning company, it is well known that oven-to-table ware is produced by controlling the firing curve to achieve the development of the beta spodumene phase in the porcelain microstructure. 'Pyrex' ware is a low expansion high silica borosilicate glass.
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- (Materials)
Spodumene - Li2O.Al2O3.4SiO2 or LiAl(Si2O6)
Crackle glaze, craquele
A type of glaze that is intentionally crazed. Stains and other colorants are often rubbed into the crack lines to heighten the effect. Crackled glazes typically severely weaken ceramic ware, especially if it is thin, low fired or porous.
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Crank, plate setter
A special kiln shelf that has legs (usually three) and stacks by interlocking with others. Cranks are used to fire plates and tiles (one per crank). They are employed to overcome poor use of space in kilns when trying to fire flat objects.
Crawling
A condition where fired glaze separates into clumps or islands leaving bare clay patches showing in-between. More prevalent in once fired ware. There are many causes for crawling (typically glazes shrink too much during drying and don't have a good bond with the bisque).
Some times glazes are made to crawl intentionally. One technique to make this happen is to add 15-20% magnesium carbonate (testing required to determine amount) to a low fire transparent glaze.
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- (Troubles)
Glaze Crawling
Asking yourself the right questions to figure out ...
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Pictures
Example of glaze crawling on the inside of a stoneware mug
Example of two crawling glazes, on the left at cone 04 on a terra cotta body, on the right at cone 6 on a porcelain
Light magnesium carbonate in a low temperature terra cotta white glaze induces crawling
The glaze on the right is crawling at the corner because the angle between the wall and base is sharper than on the left.
Badly crawled glaze fired at cone 5 reduction. It has been spray applied on the dried bowl (no bisque fire) an is too thick (not to mention underfired).
Crazing
Small hairline cracks in glazed surfaces that usually appear after firing but can appear years later. It is caused by a mismatch in the thermal expansions of glaze and body. A glaze of higher expansion shrinks more than the clay to which it is attached and therefore crazes.
There are many treat-the-symptoms approaches to crazing but the bottom line is: If there is a thermal mismatch it will reveal itself sooner or later no matter how you adjust firing or glaze thickness to hide the problem. If crazing is visible, it is an indication of a significant problem. This is because long before crazing becomes visible, serious strength problems result where glaze and clay are not expansion-compatible. In addition, crazing also call into question the functional safety of ware (e.g. bacterial hazards).
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Pictures
Crazing in cone 10 reduction celadon glazes is common because they are high in K2O/Na2O
Example of crazing in a glaze
Cristobalite
A crystalline form of silica (quartz is also) available as a raw material and formed by natural processes during firing of certain bodies. During cooling cristobalite changes from beta to alpha form around 220C. This change is accompanied by a 3% volume contraction.
Cristobalite forms spontaneously at temperatures above 1100C from very fine quartz found in some clays, from finely ground silica, and from molecular silica liberated during the formation of mullite from kaolin. This formation is a menace to most because its side effects make the body susceptible to dunting at 220C (cooling cracks, cracking during use due to sudden cooling). If feldspar is present in the body then any available molecular silica is taken up in the formation of silicates, and thus cristobalite does not form. If it does then it too is taken into solution. A good strategy in formulating a body is to use enough spar or naturally fluxed clays to be sure that any potential cristobalite is drawn into body glass (check with dilatometer test) and then re-establish fit with fine quartz. In this way quartz is compressing the glaze at 573C rather than cristobalite at 220C. A typical cone 10 porcelain with 25-30% feldspar will show no evidence of cristobalite on its expansion curve (as measured in a dilatometer). Conversely, high iron often non-vitreous stoneware bodies can generate high cristobalite levels.
A classic way to recognize a raw material (e.g. a ball clay) that forms significant cristobalite on firing is to note any significant shivering that occurs with a typical stoneware glaze. Some clays generate so much cristobalite that they will literally shed all of their glaze during final stages of cooling.
As a raw material added to earthenware bodies, cristobalite improves craze resistance after glazing because the sudden contraction puts the glaze into compression. Talc contains mineral species that, when added to earthenware bodies, act as a catalyst to the natural formation of cristobalite. This approach is necessary in low temperature ware because quartz inversion at 573C typically finds glazes still somewhat fluid, having not reached their set point (quartz inversion is used to advantage to put high temperature glaze in final compression).
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Crystalline glazes
Crystals can form during cooling and solidification in many kinds of glazes and they can be microscopic or very large, widely scattered or completely covering. Matte glazes are often such because of a dense mesh of crystals growing on the surface. Unwanted crystallization is called devitrification. However the term crystalline glaze generally refers to the pursuit of large macro crystals. People are captivated by them because they often seem to float on the glaze and they wrap to match the contour of the object. They can be of incredible size and beauty and have been demonstrated in infinite colors, shapes and patterns. But they only grow if the right conditions are present:
The chemistry: Glazes must have almost zero Al2O3 to produce a melt so fluid that it literally runs off the ware. In such glazes it is easier for the component oxides to migrate to the site of formation and they have more freedom to arrange themselves in crystal formation. A saturation of ZnO is also required, this is the magic crystallizing oxide. Adequate SiO2 is needed to form zinc-silicate crystals.
The time and temperature: Glazes prone to crystallization have a distinct "zone of crystallization". For the best results slow the firing at the peak to make sure all materials are fully dissolved in the melt and then cool to the point where the crystal forming material precipitates out into crystals and hold. Experience reveals at what temperature they grow best and how long to hold. An accurate electronic kiln controller is must to make results repeatable.
Most crystals are a different color than the surrounding glaze area (it is reduced in crystal forming oxides and is thus a 'depletion zone'). Larger crystals grow at the expense of smaller ones in a 'survival of the largest' situation. Crystals demonstrate the phenomenon of phase separation, where a glass melt separates into two or more liquids. Coloring materials tend to preferentially and selectively gather at one of these, (one coloring oxide colors the crystals, another the glassy areas). Crystal formation is actually a mechanical imperfection in the glass since it is disrupting the homogeneity of the matrix and imposing discontinuities between glass and crystal phases.
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Pictures
Example of a crystalline glaze
Cutlery Marking
In glazes with this fault rubbing a metal knife or spoon on the surface will leave black marks that cannot be completely rubbed off. This is a common fault in glazes, especially matte glazes. Even commercial tableware often exhibits this problem. It happens because the micro-surface of the glaze is not smooth and has angular protrusions that are actually abrading the metal, taking off tiny bits. Micro-crystalline surfaces will do this. Also, glazes having a high zircon content canb do this (because the zircon particles have sharp corners and edges and can protrude from the surface). Of course, glazes that are not completely melted with also have a rough micro surface that will mark easily. Dealing with the problem is usually a matter adjusting chemistry in the light of understanding the mechanism of the matteness or rough micro surface.
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Pictures
Example of a cone 10 transparent that is running severely on a flow tester, but does run on actual ware. The glaze is cutlery marking (therefore lacking hardness). This, the running and likely leaching are due to extremely low SiO2, Al2O3 content.
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