1.Anadolu University, Department of Materials Science and Engineering, Eskişehir
2.Fosfortek Phosphor Technologies Co., OSB, Aksaray
3.Anadolu University, Department of Chemical Engineering, Eskişehir
The first records of crystalline glazes being made are from the Orient, mainly China, many centuries ago. The Sung Dynasty (960-1279 AD) produced ancient Chinese oil spot glazes that contained small crystals in them, although growing the crystals was unplanned and unintentional. Later in 13th century China, during the Ming Dynasty, crystals were again accidentally formed. As far as written records show, there are not any other attempts to continue crystalline experimentation in the subsequent dynasties. During the Art Nouveau movement, near the end of the 19th century, the style of a single glaze on a simple form began to have an appeal. Oriental glazes were being imitated and crystalline glazes, with their subtle colour changes, fit into the natural and sensuous lines of Art Nouveau. Industrial ceramics and European potteries were making many stylistic and technological advances at that time, and the race for production and experimentation began. The crystalline glaze was seen as something new, and possibly profitable, if it could be refined and controlled. Although production and advancements were made by Europe into the first decade of the 20th century, nearly all crystalline work stopped at the beginning of World War One. The last and current big step in crystalline glazing is credited to the advancement of technology and the explosion of interest by the studio pottery movement. Studio potters have been using the new knowledge and technology for the last 50 years to experiment with crystalline glazes. The invention of the computerized kiln in the early 1980’s changed crystalline glazing forever. .
Fig. 1. Beautifully design, good cobalt coloured vases with large and attractive crystals on the surface [2-3].
MAKING CRYSTALLINE GLAZES
If a glaze contains proper ingredients it can form zinc crystals. The process is follows: As the temperature of glaze is increased, all the components begin to melt together. When the glaze is at the proper temperature, “seeds” begin to form in it. As the glaze reaches its maximum temperature, it begins to flow, and many of the seeds dissolve. The kiln temperature is then lowered. When the temperature reaches the correct range, the remaining seeds acting somewhat like magnets attract appropriate minerals in the glaze and the crystals grow on the seeds. The longer the temperature is held, the larger the crystals grow.Zinc crystalline glazes can be formulated to fire from cone 3 to cone 12. Those firing at cone 8 and above are frequently called high-fire glazes. In general they give the nicest (largest and most interesting) crystals. But for potters not able (or willing) to fire that high, there are also several low-fire (cone 3 to cone 7) possibilities. Must you use an electronic kiln controller when firing crystalline glaze. Some feel that convenience and reproducibility that controllers offer are a real advantage. Others feel that they make the process to “mechanical” and remove much of the spirits randomness from the experience . Producing these glazes can be technically difficult. Diane Creber tackles the problems encountered in using this challenging medium. It used to be thought that crystalline glazes were only possible in an oxidation atmosphere. But as many more potters become seduced by these intriguing glazes, new and exciting discoveries–including developing crystals in reduction–are being made all the time. In today’s post, crystalline potter Diane Creber have been recently experimenting using reduction to enhance the pre-formed crystals in their glazes .
Fig. 2. (a) Bottle with gold stuff glazed and fired in oxidation, then refired to 1500 oF and reduced until the kiln cooled to 1250 by Diane Creber, (b) Covered jar with gold stuff glaze, fired in oxidation to grow crystals, then reduced from 2000-1850 oF by John Tilton, Alachua, Florida . As the glaze is matured and cooled in the kiln, molecules bond together in random strings. Crystals occur if the glaze is fluid enough to allow molecules to move freely and fastly and stay long enough in certain temperature range to allow the glaze molecules to arrange themselves in structured strings, or crystals. It is to the potter’s advantage to use a frit. Then, a crystalline glaze could consist of only three ingredients: frit, zinc oxide, and silica. This combination, in the right proportions, will produce a crystalline glaze. Almost all of the zinc oxide bought today is calcined. The silica should be 400 meshes (used for most glazes) because this particle size makes it easier to get a complete melt and no nucleation (the beginning of crystal growth, based around an unmelted zinc-silicate crystal). Very small amounts of alumina or bentonite are often added to a glaze to help keep the materials in suspension (1–2 %). These ingredients must be in very small quantities or the glaze will opacify and become a matte one. Adding a small amount (1 %) of Epsom salts to the glaze keeps the glaze flocculated .
Fig. 3. Some good quality, crystalline glazed ceramic samples with different colouring agents .
IMPORTANCE OF ZINC OXIDE IN GLAZES
Zinc oxide is a fluffy white to yellow white powder having a very fine physical particle size (99.9 % should pass a 325 mesh screen). Ceramic grades are often calcined to remove any physical water (so they do not clump in the bag). While calcined grades are said to produce less glaze surface defect problems, many ceramists have used the raw grades for many years without serious issues. One can calcine (or re-calcine) zinc on his/her own in a bisque kiln at around 815 oC. However, there are issues: First, because the calcined zinc wants to rehydrate (and get lumpy in the process) it needs to be stored in an airtight container (some people calcine a mix of zinc and kaolin to prevent this, but theoretically such a mix should not even require to be calcined). Second, the calcined zinc may produce slurry thickening issues over time. Lots of zinc is used in crystalline glazes (typically 25 %), because these have no clay content. The raw zinc suspends glazes better (the calcined settles out significantly more). The raw zinc takes more water, but since the glaze can thin out over time it is better to add less than needed at mixing time and mix thoroughly. Zinc oxide is soluble in strong alkalis and acids. It can be an active flux in smaller amounts. While boron dominates as the key flux in middle temperature glazes, for example, zinc is employed in some base glazes to augment the B2O3 or even replace it entirely . Zinc is a component of an amphoteric nature, which under certain conditions can replace silicooxygen tetrahedrons in the network, forming a tetrahedron (the intermediate component). Zinc may also be present in the form of octahedrons and act as network modifier . No combination of the common raw materials feldspar, kaolin, silica, feldspar, calcium carbonate, dolomite and talc will melt properly at cone 6; however, a 5 % addition of zinc can transform the mix into a glossy glaze. 5 % more and it will be a very fluid glossy glaze. The zinc can also significantly reduce the thermal expansion of the glaze. If too much is used the glaze surface can become dry and the heavily crystalline surface can present problems with cutlery marking. Other surface defects like pitting, pin holing, blistering and crawling can also occur (because its fine particle size contributes to glaze shrinkage during drying and it pulls the glaze together during fusion). While it might seem that zinc would not be useful in reduction glazes, when zincless and zinc containing glazes are compared it is often clear that there is an effect (e.g. earlier melting, more crystallization and variegation). .
Fig. 4. Zinc oxide calcined (left) and raw (right) in close up of a crystalline glaze (contains 25 % zinc) with high expansion (low crazing) body [9-10]
COLORANTS IN CRYSTALLINE GLAZES
Chromium, cobalt, iron, uranium, copper, gold, vanadium and zirconium oxides are the most prominently used ceramic colouring agents. To be suitable as a ceramic colour, a material must have strength of pigmentation and stability. The colour of an atom depends on its environment. This is particularly true of chromium. Chromium compounds normally are green, but in combination with tin and Al2O3, pinks and red are produced. The influence of environment is far less pronounced with cobalt, which similar to most other colouring elements, undergoes changes in shade. However, cobalt does not give completely different colours, as does chromium, if exposed to various environments. Crystalline glazes containing up to 10 % CoO remain blue. Bodies and engobes can carry more cobalt than glaze without becoming black. CoO is an essential component of the best black ceramic pigments. Normally, cobalt is not volatile, even at the highest glazing temperatures (1400 °C), but, in the presence of chloride ions, it is volatile, and blue coloration strikes onto other nearby glazed surfaces. Copper, although rarely stable at high temperatures, is similar to chromium and iron in that it produces different colours under various conditions. Normally, copper is used for green shades at low temperatures. In highly alkaline lowtemperature glazes, copper turns to a beautiful turquoise (marine blue). The magnificent reds originally made in ancient China and reproduced in Europe since the 19th century, known as rouge flambé and sang-de boeuf, originate from copper and are only obtained under reducing atmospheres introduced at the correct stage .
Fig. 5. Some objects showing the effect of colouring agents in crystalline glaze [12, 3].
FIRING CRYSTALLINE GLAZES
Crystalline glaze firing has very special requirements. One needs to heat rapidly and cool rapidly plus needs to be able to hold high temperatures for long periods of time to grow the crystals. Crystalline glazes are often fired to around 2300-2400 °F or cone 10-11. Computer controllers, modified to ones specifications, are typically used to carry out a programmed firing. After heating the glaze to a full melt, the kiln is dropped quickly to temperatures within the range of 1825-2185 °F. At these intervals, a “heat-soak” (hold) is maintained, to initiate and form the crystals. The temperature and duration create different sizes, patterns, and colours within the crystal and the corresponding background .
CRYSTALLINE GLAZES IN THE WORLD
There are lots of potteries in the world and their wonderful works are worth seeing. Unfortunately presenting all of them in this paper is impossible. Therefore, only limited number of works will be given hereby. First, Matt Horne has unbelievable works about crystalline glazes. He started to this passion when he was 15. Some examples of his works are given in Fig. 6.
Fig. 6. Works of Matt Horne . Secondly; French Halmos who is the talented pottery in Hungary (Fig. 7).
   
Fig. 7. Works of French Halmos on zinc silicate crystalline glazes [14-17]. Next potteries were made by Bill Boyd from Sweden (Fig. 8).
  
Fig. 8. Works of Bill Boyd on crystalline glazes [18-20].Another examples from company called Wauw Design (Fig. 9)
  
Fig. 9. Samples from the Wauw Design Company [21-23].
Samples from the company called ‘Maycocolor’ are also given. Their crystalline glaze program creates opalescent crystals with more consistency, higher probability of growth and with a greater level of control over the location of crystal development. The system involves the use of three products: engobes, seeding media and crystalline clear .
Fig. 10. Samples of Maycocolor Company 
Matt Crystalline Glaze
These contain 2 types of crystals; macro ones supplying a decorative effect and micro ones covering the full glaze
surface and giving a nice shine and are smooth to the touch (Fig. 11).
Fig. 11. Samples of matt crystalline glazed objects [25-26].
Some examples of matt crystalline glazes from Glenn Woods and Keith Herbrand can be seen below.
  
Fig. 12. Matt crystalline glazed works of Glenn Woods and Keith Herbrand [27-29]. Portland Growler Company is another successful company in New York (Fig.13).
Fig. 13. Samples of Portland Growler Company .
Coaxing crystals large enough to see, like the ones on their crystalline growlers, require a very scientific firing process. Invisible crystals exist in most glazes, but to grow larger crystals need a bit of a «perfect storm.» One should have a precise ratio of ingredients, and the firing should be conducted at a specific temperature for an extended period of time, or the crystals won›t form .
CRYSTALLINE GLAZES IN TÜRKİYE
Eskisehir Modern Art Museum
Fig. 14. A few examples from Soner Genç work done and still being exhibited in Eskişehir Modern Art Museum at Anadolu University .
Genç , in his work formed in the form of a catalogue, gave the sophistication of ceramic arts, type of mud used for art glazes, firing temperatures and milling time. Moreover, he presented some of his crystal glazed plates, samples of leather crackle and matt glazed vases.
Serant Ceramic and Materials in İzmir
Fig. 15. Works of Serant Ceramic and Materials .
SCIENTIFIC STUDIES ON CRISTALLINE GLAZES
One of the earliest studies is dated to 1937. Norton  has published a paper on the control of crystalline glazes. Before 1976 it was reported that except rutile based glazes their usage was not easy in the production of ceramic tiles. Normally one has to use very slow cooling rate to produce them. However, with the modern industrial furnaces employed in ceramic field this is not always possible. There are several ways to achieve successful crystallization: by either leaving the glaze to be slowly cooled down after maturation or at the beginning of cooling stage by supplying required furnace environment in the formation of calcium and zinc silicate crystals . Karasu et al. [11, 36-48] have conducted studies on soft porcelain zinc crystal glazes in which the effect of certain colouring agent, like CoO, CuO, MnO2 and TiO2 was investigated. Many artistic effects were achieved by departing from a clear, smooth, transparent system. Coloured glazes were produced by several means such as the inclusion of colouring oxides, addition of stains, dispersing finely divided particles and the use of precious metals, applied in the form of lines or bands, or even screen-printed patterns. In porcelain production, as well as bodies expected to have white colour, compaction and translucency, suitable glaze compositions have great importance from both a technological and decoration point of view. Supplying the desired firing conditions is generally quite difficult for the porcelains having white colour after being
fired in reducing atmospheres.
Fig. 16. The microstructures of 0.3 % CoO added zinc containing soft porcelain glaze treated at 1180 oC (a) and (b) showing the formation of rods (willemite) and star-like shaped crystals (gahnite), after heat treatment at 1080 oC (c) and (d) with star-like gahnite crystals in willemite matrix . Dvornichenko and Matsenko  searched for the production of crystalline glaze with a decorative aventurine effect based on frit containing Na2O, B2O3, and SiO2 using the simplexlattice method of experiment design. It was established that the aventurine effect is caused by hematite crystals. Rudkovskaya and Mikhailenko  published data on compositions and properties of zinc-containing crystalline glazes for ornamental ceramics. Knowles and Freeman  made investigation on the crystalline glazes on ceramic plates produced commercially in the U.K. and on ceramic pots produced commercially in Taiwan and Spain. They observed that the macroscopic two-dimensional spherulites within the glazes clearly seen by the naked eye were found to consist of large numbers of radially orientated acicular crystals each 5 μm or less in width embedded within the silica-rich glaze. In addition to willemite, small iron-doped gahnite (ZnAl2O4) crystals were found in a honey-coloured crystalline glaze and acicular rutile (TiO2) crystals were found in the Portmeirion Pottery plates examined. Transition metal ions with a preference for tetrahedral coordination were observed to substitute for Zn2+ ions in willemite and to partition preferentially to the willemite crystals, whereas ions preferring octahedral coordination preferred to remain in the glaze.
Fig. 17. (a)–(e) Reflected light micrographs of spherulites respectively. The scale bar for all the micrographs shown in (b) is 5 mm. The spherulites in (a) are at the edge of an as-received broken piece. This edge runs from the middle of the right-hand side of (a) to near the bottom left-hand corner of (a) .The effects of ZnO addition on the crystallization behaviour and mechanical properties of porcelain bodies have been investigated by Lee et al. . When ZnO was incorporated, gahnite phase crystallized from 1130 ◦C, after feldspar had significantly melted and ZnO dissolved into the glass. Cristobalite formation was promoted with the addition of ZnO. However, alumina used as a raw material remained almost intact during sintering. An abundance of gahnite crystals with sizes of 50–400 nm were observed inside the glassy phase, which might contribute to the improved mechanical properties of the porcelain bodies. The flexural strength depended more considerably on the amount of ZnO, in comparison with water absorption and sintering temperature. Wear resistance was enhanced with the addition of ZnO. Göncü and Ay [53-54] aimed to study industrial evaluation of crystalline glazes, which are very valuable from the artistic point of view. Industrial waste was used instead of ZnO in zinc crystalline glazes and its decorative effect on granite and stoneware bodies was examined after having obtained the most suitable heat treatment conditions. It was seen that the nucleation and covering abilities of industrial waste are better than that of pure zinc oxide. The waste containing copper gives green colour to the glaze without a need of other colouring agents and therefore, the industrial waste can be used in zinc crystalline glaze recipes as a substitute for zinc oxide. Xia et al.  conducted the study on the use of crystalline of calcium carbonate (CaCO3) addition into glaze batches on the crystallization behaviour of crystal glaze. They identified these crystals as willemite (Zn2SiO4) in the form of spherulites. SEM analysis indicated that willemite crystals are in the acicular needle-like shape. XRD result showed that the intensities of crystal peaks decreased with the addition of CaCO3 up to 3.0 wt. %. However, there was no willemite crystals formation as the amount of CaCO3 raised to 5.0 wt. %. Cobalt-doped willemite is a promising blue ceramic pigment, but some important aspects concerning crystal structure, optical properties and technological behaviour are still undisclosed. In order to get new insight on these features, willemite pigments (Zn2−xCoxSiO4, 0 < x < 0.3) were synthesized by the ceramic route and characterized from the structural (XRPD with Rietveld refinement), optical (DRS and colorimetry), microstructural (SEM, STEM, TEM, EDX, EELS) and technological (simulation of the ceramic process) viewpoints . The incorporation of cobalt in the willemite lattice, taking preferentially place in the Zn1 tetrahedral site, induces an increase of unit-cell parameters, metal–oxygen distances, and inter-tetrahedral tilting. Willemite pigments impart deep blue hue to ceramic glazes and glassy coatings with a colouring performance better than commercial Co-bearing colorants in the 800–1200 ◦C range.
Fig. 18. Nicely grown crystals .
Jamaludin et al.  published data on the ceramic samples fired at different gloss firing temperature of 1200 to 1250 oC for half an hour and hold for five hours at crystal growth temperature. The result exhibited crystals start to develop at 1230 oC of gloss firing temperature. SEM analysis indicates that these crystals are made of acicular needle shape crystals. XRD and EDX analysis of the phases identified these crystals as willemite, which formed spherulites. The results also inhibit obvious growth and distance distribution between each of spherulites as the gloss firing temperatures increase to 1250 oC. A willemite brown inorganic pigment was synthesized by substituting MnO for ZnO . The composition of MnxZn2-xSiO4 (X=0.1, 0.3, 0.5, 0.7 and 0.9 mole) was synthesized at 1200~1300 °C by solid state
method. Willemite single crystal phase was synthesized with compositions of Mn0.3Zn1.5SiO2 and Mn0.5Zn1.5 SiO2 from 1300 °C/3h. The synthesized pigment of Mn0.5Zn1.5 SiO2 at 1300 °C/3h was applied to lime barium glaze and lime zinc glaze and the CIE L* a* b* values were 30.32, 7.17, 3.14 and 32.04, 8.18, 3.49, respectively. Silakate et al.  had the objectives of preparing leadless crystalline glazes from iron oxide by using low temperature firing (1100 °C) and to study the effect of concentration of iron oxide on the phase composition of the glaze raw materials on phase transformation in leadless iron oxide crystalline glaze. The composition of the glaze raw materials compose of nepheline syenite, colemanite, pottery stone, bentonite, ZnO, Li2CO3, SiO2 and 10, 15 and 20 % (w/w) iron oxide (Fe2O3). It was found that the crystallization temperatures (Tc) of franklinite (ZnFe2O4) and anorthite (CaAl2Si2O8) depend on the concentration of iron oxide content. The effects of calcium carbonate addition on the physical properties of ZnO-based crystal glaze batches were investigated by Jamaludin et al. . Samples were fired at different gloss firing temperatures ranging from 1180-1220 °C with 3 hours soaking at 1060 °C crystallization temperature. XRD analysis identified the crystal phase occurred as willemite and SEM indicated that willemite crystals are in the acicular needle-like shape that formed spherulite. The intensities of willemite peaks decreased with CaCO3 addition and completely vanished at 5.0 wt. % CaCO3. Liu et al.  made a search on the zinc silicate crystallineglazeusing the sintering temperature of 1100°C with zinc oxide as seedcrystal. The mechanism of zinc silicatecrystalgrowth and crystallization was discussed according to thecrystaldynamics theory. The results showed that thecrystalformation rate increased significantly with time when the soaking time was over 30 min, and thecrystalformation rate reached the maximum value when the soaking time was in the range of 45-60 min. When the soaking time was over 90 min, thecrystalformation rate markedly decreased with time, and thecrystalgrowth rate basically reached a stable value when the soaking time was over 150 min. Using zinc oxide as seedcrystal, zinc crystallineglazewith good performances and stability can be prepared. The promotion of zircon (ZrSiO4) crystallization by ZnO from a zirconium-based frit glaze was studied and the possible mechanism was discussed by Wang et al. . The results showed that ZnO can significantly decrease the crystallization temperature of zirconium-based glaze, depress the formation of Ca2ZrSi4O12, and promote the devitrification of transitional crystals t-ZrO2 and Ca2ZnSi2O7, as well as lead to the formation of more zircon than the ZnO-free glaze. It was also found that zircon not only can form from the interaction between t-ZrO2 and SiO2 but also can devitrify directly from the glass phase of zirconium-based glaze. Zinc crystal glaze has its limits in practical use of commercial glaze due to the sensitive firing schedule. In order to overcome these limits, and to improve the practical usage, Lee et al.  made a study aiming to develop a stable zinc crystalline glaze which altered the quantity of nuclear formation of zinc crystal glaze in order to control the willemite formation in the glaze. The addition of ZrO2 to zinc crystal glaze influences the quantity of nuclear formation and its preservation; thus ZrO2 was used to control the optimal firing temperature and the size of the crystal formation in the glaze to find a zinc crystal glaze capable of withstanding various ranges of temperatures. The crystals form by slowly cooling the glaze to allow a few large crystals to grow. The growth is interesting because the glaze is typically only ~0.5 mm thick and the crystals must therefore form as platelets. A seed of TiO2 is usually used to nucleate the crystal in a low-viscosity glaze, giving what is termed “rutile breakup,” which is actually the formation of PbTiO3. The chemistry of the glaze is thus important, with SiO2 and Al2O3 being low and PbO at 8–10 wt. %. The growing crystal tends to incorporate Fe from the glaze but can also preferentially exclude other dopants. Modern potters tend to use Zn as the modifier and produce willemite crystals. The technique is tricky because the addition of large amounts of Zn (a network modifier) to the glaze causes its viscosity to remain low even at low temperatures, so that it tends to run off the pot! Each spherulite is actually a mass of radiating crystals that are similarly aligned with respect to the centre of the spherulite (Fig. 19) .
Fig. 19. Spherulitic crystallization in a glass .
Zinc crystal glaze has its limits in practical use of commercial glaze due to the controlling crystal. In order to overcome this limit, and to heighten the practical usage, Lee  aimed to develop artificially controlling willemite zinc crystalline glaze. For this purpose, he has experimented with the effect of anatase form and rutile form using TiO2 known as nucleating agent. In zinc glaze, adding TiO2 resulted with anatase form becoming more effective at nucleating formation and growth of willemite than the rutile form. Furthermore, it turned out that using the TiO2-anatase form, with synthetic seeds (zinc silicate), the numbers and positions of crystals can be controlled artificially. The family of M(II) substituted ZnO– wurtzite was investigated as potential new ceramic pigments by Araceli and Griselda . When MxZn1-xO or (MM›)xZn1-xO with (M/M0¼ Mn, Co, Ni, Cu) oxides are incorporated to industrial frits the development of new colour huesis observed. These materials are promising for ceramic applications since a low amount of pigment would be necessary to achieve intense colouration. Sirijan et al. aimed to investigate crystallization of willemite phase
in ceramic glazes. XRD revealed three different phases (willemite, anorthite and quartz). Large crystals were examined by SEM and were found to be composed of much smaller lath-like crystals. The kinetics of crystal growth was examined via the grain growth kinetics. Currently, diopside (MgCaSi2O6) crystal glaze is used frequently for pottery works or in earthen wares, though the process is not straightforward. However, to create and control the positions and sizes of the crystals in desired amounts when making pottery is difficult. To solve this problem, a diopside crystal seed was created at a temperature of 1450 oC . After planting this seed in the glaze, a glaze combination and firing process which allows a user to create crystals with the desired position and at the desired size were established. As a result, the optimum synthesis condition of the diopside seed was created by mixing 1 mole of CaCO3, 0.2 mole of(MgCO3)4(MgCOH)2•5H2O and 2 moles of SiO2 and then applying a firing process to the mixture at 1450 oC for 30 minutes. The optimum glaze content of the seed was 70 % feldspar, 20 % limestone and 10 % MgCO3. For the firing process, it was confirmed that the size of crystal is larger with a longer firing time at 1100 oC by completing a two-hour process at 1280 oC. In addition, the diopside crystal has columnar structure and is less than 1 μm in size. Tabrizian et al.  prepared crystalline glazes based on the willemite crystals in the presence of nickel, titanium and iron oxides by two step heating process, i.e. glaze firing at 1200 oC and crystallization above differential thermal analysis crystallization peak temperature. The result showed that titanium and iron oxides induced a bulk crystallization mechanism in the glaze while in the presence of nickel oxide crystallization of willemite was initiated from the surface layer. Willemite got crystallized randomly as spherulites in the nickel bearing glazes when it was applied on a horizontal substrate. However, it was crystallized as grape-like clusters in the vertical surfaces. Crystalline glaze has micro or macro crystals inside or on the surface of glass phases where macro crystals occur being embedded inside the glaze and micro ones form as bunches on the glaze surface. The smallest element forming these bunches is called core, coming together in melted glaze and forming macro crystals. Pekkan et al.  intended to investigate the effect of ZnO addition on crystalline formation and development inside the fritmixtures prepared in various ratios. Willemite crystals are formed during glaze firing cycles instead of frit preparationstep. Successful crystal glaze surfaces are obtained by fritted recipes of ZnO-Na2O-SiO2 (ZNS) and ZnO-Na2OSiO2- Al2O3 (ZNSA) glaze systems. Lee  made an investigation on the colorization of Zn2SiO4 crystal glazes by adding nucleating seeds with various colouring agents. The addition of colour fixing agents such as Fe2O3, MnO2, and NiO with seeds caused changes in the colours of glazes.
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[ 9 ] http://digitallfire.com/4sight/images/materials/wotwuhalyp.jpg
[ 1 0 ] http://digitalfire.com/4sight/images/glossary/jujsehucuc.jpg
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[ 1 2 ] http://www.matthornepottery.co.uk/uploads/2/3/8/8/23883384/6918483_orig.jpg http://hotkilns.com/
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[ 1 4 ] http://www.puttgarden.com/crystal/friends/halmos/8-20-07/c0112.jpg
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[ 2 1 ] http://www.wauw-design.dk/#!product/prd12/4205043721/large-crystal-glaze-vase%2C- darkbluekobalt
[ 2 3 ] http://www.wauw-design.dk/#!product/prd12/4205042891/large-crystal-glaze-vase%2Crosa-antique
[ 2 4 ] https://www.maycocolors.com/index.php/colors/crystalline
[ 2 6 ] http://www.puttgarden.com/crystal/friends/halmos/8-20-07/c0123_resize.jpg
[ 3 0 ] http://portlandgrowlercompany.com/collections/crystal-glaze
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