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Bacteria, Mold on Clay
Over time mold can grow on pugged clay stored in bags. The amount of growth depends on temperature and available light, time and type of clay. Porcelains or high iron clays grow less, coarse clays grow more.
Pictures
Amazing mold (actually sprouting leaves) that has grown on pugged clay after 10 months of storage where there is sunlight.
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Mold that has grown on pugged clay in a bag.
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Ball milling
A device used to reduce the particle size of materials, bodies or glazes. A ball mill is simply a container that is filled with pebbles (either of porcelain or stones e.g. Flint) into which a charge (powder or slurry) is put and that is then mechanically rotated to cause the tumbling pebbles to crush particles that happen between them. Ball mills can be continuous or periodic, they can be small or gigantic, low speed or high speed, rotated or vibrated or both. For maximum efficiency a ball mill should be made of, or lined with, a porcelain or other very hard surface (so grinding also occurs between the wall and the balls), the balls should be of a range of sizes (to maximize points of contact), the mill should have the correct quanity of balls and the charge should be an optimal amount (over charging reduces efficiency). Various compromises are often made (for example rubber lining mills to reduce wear and noise).
Ball mills can reduce particles to the nano sizes, the process is very important in creating powders used in hi-tech industries. Ball mills are slow compared to other methods of grinding, it could takes hours, for example, to grind all the particles in a clay to minus 200 mesh. In fact, if mica or other flat particles exist it may be practically impossible to grind them to minus 200, for this reason ball milling is normally done in consort with wet screening and/or roller-milling/air floating, for example, so that large particles have already been removed by the time the material reaches the ball mill or can be screened out after milling. Air floating can also be done in consort with dust ball milling. The milling process can also reduce particle sizes by too much for an application, so a means of measuring the distribution of ultimate particles is important to be able to set the parameters for the process.
A clay body that has been ball milled will be more plastic, potentially much more plastic. Ball milling of the body or selected body materials will reduce or eliminate many types of fired glaze imperfections (especially specking, blistering and pin-holing. Milling a glaze will also produce a cleaner fired result with less imperfections. Milling of slurries presents less technical challenges than dust milling, but if dewatering is necessary it may be impractical. A simple ball mill can be constructed by almost anyone, but obtaining the hard pebbles with the correct range of sizes for inside the mill can be challenge (they are expensive).
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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.
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Ball mill rack front side
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DFAC tests of Plainsman A2 ball clay at 10 mesh (left) and ball milled (right). The finer particle size greatly increases shrinkage.
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Bamboo Glaze
Usually describes a bamboo colored matte glaze (especially in reduction stoneware). These are normally achieved in a magnesia white matte by adding a small amount of iron (0.5-1%) and possibly some tin oxide opacifier.
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Pictures
Bamboo glaze made by adding 4% tin and 0.5 and 1% iron oxide to G2571A at cone 10R
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2571A base glaze with 4% tin and 0.5 and 1.0% iron oxide to create a bamboo matte
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Basic
See Acidic.
Bat
Many people in Europe use the term 'bat' to refer to kiln shelves (i.e. UK). Thus 'bat wash' is kiln wash. However in North America, 'bat' most often refers to wood, plastic, or plaster disks which are used on the potters wheel. Bats are held in place by pins, an interlocking arrangement, a sticky membrane or glued on by moist clay or slip. Ware can then be thrown on the bat and the whole thing removed to make another piece.
Batch Recipe
The term 'batch' refers to the actual mixture-by-weights of materials that you weigh out when you prepare a glaze or clay body batch for production or testing. The term 'recipe' is more correct than 'formula', the latter refers to the chemistry of the mix.
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Binder
A glaze or body additive designed to strengthen it during the dry stage or to make it more durable to withstand handling during processing. Binders enable using cleaner but less plastic materials. The mechanism of a binder can be as simple as a glue that hardens and bonds particles together. Other binders, especially inorganics, have more complex mechanisms. Body binders can also make it possible to form powders that would not otherwise be plastic enough to hold a shape. Likewise, glaze binders make it possible to use slurries with very low clay contents yet still have a hard enough surface to withstand handling. Binders come at a cost, they slow down drying and often pose issues related to their decompositions during firing. In some industries, like tile, they are either not used or only selected ones are practical (e.g. sodium silicate).
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Bisque, bisquit
The practice of prefiring ware without glaze to make it impervious to water, easy to handle, or vitrify it. Glaze is then applied and it is fired again. 'Low' bisque firing is typical for pottery and ceramics while vitrified bisque is done for bone china and some types of stoneware. Low bisque should be fired as high as possible to burn away all carbonaceous matter, yet low enough to provide enough absorbency to make glaze application easy. 'High' bisque firing is done to mature the body (i.e. bone china) and subsequent firing is usually done to apply a low fire glaze. Such glazes must have special additives to make them gel and stick to the ware (i.e. calcium chloride, gum); these glazes take much longer to dry.
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Black Coring, Black Core
Black coring usually occurs during a reduction firing and is a result of fast firing and/or lack of oxygen in the kiln between 700 and 900C (usually in the bisque firing). If body carbon fails to oxidize to CO2 it steals oxygen from Fe2O3 (reducing it to FeO, a powerful flux.) This FeO will then flux the body, sealing it and preventing the escape of remaining carbon in the body. This produces the characteristic 'black core' you see on ware cross section. The more iron in a body, the greater the risk of this problem if firing is not right. Once iron is reduced to it is very difficult to reoxidize it back to Fe2O3.
Note that electric kilns can also produce this problem, depending on the carbon and iron content of the clay, density of the pack, available airflow, and speed of the firing.
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Pictures
A test bar of ball clay fired to cone 10 reduction. Notice the black carbon core. Ball clays commonly contain carbon.
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Bloating
Bubbling that occurs in clay bodies if they are over fired. Aggravating conditions include the presence of mineral particles that generate gases during the over firing stage, the presence of excessive carbon not burned away by bisquit or oxidation firing, laminations in the clay matrix, the presence of an early melting glaze that seals the surface preventing gas escape. Many bodies do not tolerate overfiring well (e.g. manganese speckled bodies, heavy iron stonewares). Many kilns do not have reliable shut-offs or the temperature measurement devices on computer controlled kilns have deteriorated or are not accurate and users are unwittingly firing them too high. It is best to confirm firing temperature using properly set cones to avoid bloating with touchy bodies.
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- (Glossary)
Warping
Normally refers to a body problem where vessels wa... - (Glossary)
Stoneware
A high fired (about 1150C+) ceramic clay that is s... - (Troubles)
Body Bloating
Bloating in clay bodies occurs when the firing goe... - (Glossary)
Porosity
In ceramic testing this term generally refers to t... - (Glossary)
Laminations
Laminations are planes of weakly connected materia...
Pictures
An example of a terra cotta body being fired from cone 5 to cone 8. At cone 6 bloating begins to occur.
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Bloating in an over fired terra cotta body. It is OK at cone 4 it is very dense and strong but suddenly bloating begins at cone 5. Such bodies must be fired at lower temperatures to avoid this volatility.
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Cone 6 iron bodies that fire light brown or red in oxidation can be dark brown or melting and bloated chocolate brown in reduction (Plainsman M350 (right) and M2).
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Blunging, mixing
Mechanical mixing of a clay slurry. Blungers of all different types are available. Some are simply a high-speed propeller on a shaft, others employ complex arrangements of paddles and container shapes. Clay slurries require the application of considerable amounts of energy to achieve a slip in which water has penetrated well between all particles.
Pictures
Simple propeller mixer with mount and switch (this 1/3 hp mixer can handle up to 10 gallons).
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Body, clay body
A typical stoneware or porcelain body is mixture of raw and/or refined clays with possible additions of silica as a filler and thermal expansion increaser and feldspar as a flux. In vitrified bodies the larger grains of quartz and other refractory particles remain unmelted while the clays go into solution in the feldspar glass and transform into mineral forms that impart rigidity to the mass. Non-vitreous bodies have strength by virtue of whatever particle bonding has been accomplished by melting fluxes in the mix. Earthenware bodies develop much less or even no glass bonding and depend more on sinter-bonding.
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Bone China
True bone china is a special type of porcelain that is more translucent and stronger. Instead of feldspar, bone ash is used as the flux. The process requires more careful attention than normal porcelain. The vitrification range is narrow, kilns need to be fired carefully. Ware warps so badly that it must be fired without glaze and supported in setters during the firing (it is then glazed and fired again at a low temperature). Forming methods have to be adapted to the very low plasticity of the bodies. Glazes have to be adapted because the ware has zero porosity. The whole process is quite foreign to what workers in an average pottery would be accustomed to.
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Borax Frit, Boron Frit
This term is very generic, referring of course to frits that contain boron. Unfortunately that is 80-90% of available frits! Boron frits may have 1% boron or 50% boron. Even though the boron in the frit is no longer in the borax form it is still customary to refer to such as "borax frits". Since many textbooks call for a borax frit in certain glaze recipes it can be very difficult to determine what frit to use. However many technicians agree that the common frit formula of Ferro 3134 is what is intended by the generic term 'borax frit'. This frit is not a glaze-by-itself like 3124 or 3195, it is intended to melt at a very low temperature and as a way to add borax without alumina.
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Boron Blue
Boron blue is the bluish haze or clouding in a transparent boron glaze that usually results from the crystallization of calcium borate. This is a common problem in borate glazes, the higher the boron the worse. It can be reduced by increasing the amount of Al2O3 in the recipe to stiffen the melt (the crystals grow in a fluid melt) and/or using no more boron than is necessary to get a good melt. Of course, for the crystals to grow, sufficient CaO must be available and the kiln must cool slowly enough; therefore addressing either or both of these situations can also reduce the problem.
Boron blue can also be used as a decorative effect, especially on low temperature ware.
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Opacifier, Opacification
A glaze additive that transforms an otherwise tran... - (Glossary)
Transparent Glazes
A fully transparent glaze is simply one that does ... - (Glossary)
Opacity
Ceramic glaze opacity refers to the degree to whic...
Pictures
This high boron cone 04 glaze is generating calcium-borate crystals during cool down
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Boron, Borate
The term 'boron' of course refers to the oxide B2O3 in glazes. However in the ceramic industry it has a more general meaning. Glazes that fire to approximately cone 8 or less will not melt enough using normal high temperature fluxes like Na2O, K2O, CaO and MgO (Li2O and ZnO are powerful auxiliary fluxes but do not make a system flexible enough to create many glazes. Thus the entire ceramic and enamel industry relies on the flux boron to make glazes melt at lower temperatures with a manageable thermal expansion, it is comparable to lead in fluxing power (and of course much safer). Thus the term 'boron glaze' is another way of saying 'a glaze that melts below stoneware temperatures', because boron is the only practical way to do that (in past years lead compounds worked even better than borates, but of course they are toxic). Boron is a wonderful material because it is a glass (can substitute for SiO2, at some loss in hardness of course, and combines readily with bases and colors) and also a flux, it has a low thermal expansion and it is non-toxic. Boron is not a common mineral on planet earth. Most of the worlds reserves are in Turkey, but also in the US, Chile, Russia. Insoluble boron is sourced by frits, colemanite, ulexite and gerstley borate.
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Borosilicate
A silicate is an SiO2-centric solid (crystalline or glass). A borosilicate simply is a silicate with boron. The term 'borosilicate' is synonymous with medium and low fire glazes because boron is not employed at high temperatures (CaO, Na2O, MgO, etc flux silica and bond with it to form crystalline or glass solids on cooling). The term 'borosilicate' almost always refers to a frit in ceramics. Such frits may have 5% B2O3 or 50%, but they always have plenty of SiO2 so the two can combine, and with other oxides in the melt, form silicates. Since boron is the melter that makes the glaze or glass possible, terminology referring to the chemistry normally mentions it.
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Buff stoneware, buff ceramicclay
'Buff' is a term used to describe the color of a non-vitreous or semi-vitreous fired clay that has enough iron to take away its claim to being white yet not enough to make it a brown or red burning ceramic. Buff coloration is generally a straw color or yellowish white. Grey-white firing bodies are not usually referred to as 'buff' firing because the grey coloration is associated with vitrification, especially in reduction.
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