INTRODUCTION
Jun (or Chün) wares have long fascinated me. Every time I go to London, I make a pilgrimage to the Victoria & Albert Museum to see them, and delight whenever I see them elsewhere. Their luxurious, opalescent surfaces captivate my imagination and make me want to try to create similar glazes. Many commenters have said they have the “appearance and feel of finest jade” (Kingery & Vandiver, p. 1269). Jade was all the rage in Song Dynasty in China, so this may have been the goal of the potters.
These glazes were long a mystery, and no one knew exactly how they were produced. We are lucky to be alive now! Modern academics and potters alike have run extensive tests and come up with some answers.
I have read all the accessible resources I could find about these glazes, and in this blog, I lay out the pertinent findings. This endeavor, to understand Jun glazes, is primarily to help guide my own glaze experiments with local materials. I warn you, it is a long blog post, but that’s what demystification takes sometimes!
FIRSTLY THOUGH, WHAT ARE JUN GLAZES?
Nigel Wood, author of the outstanding book Chinese Glazes, says Jun wares are characterized by “heavier bodies, and unusually thick bluish glazes” (Wood, p. 118). But rather than go into a long description of these glazes and their variants, it’s probably easier to show you, so here are some examples from the Victoria & Albert Museum…
As you can see from these pictures, and the ones below, these glazes vary a huge amount. Their cloudy, opalescent colors can vary in color from green to sea foam, to aquamarine, to darker blues, lavender, white, and all of the tones in between these. The surface of these glazes are dimensional, meaning there are hidden depths. They are thick and curdled, but feel like satin. They often have pin holes or pits, and sometimes strange “earthworm tracks” or lines that meander across them.
Here are some more lovely examples, from the Freer Gallery in Washington D.C…
WHEN AND WHERE ARE THEY FROM?
Jun wares originated in China, in Henan Province, during the Song Dynasty. Here’s what Sotheby’s (the auction house, which occasionally sells these pots) says, “‘Jun’ ware, with its type site represented by the Juntai kilns in the former region of Junzhou, modern-day Yuxian, Henan province, was produced by many different manufactories in Henan, including the Ru kilns at Qingliangsi in Baofeng, probably from the end of the Northern Song period (960-1127AD) until at least the Ming dynasty (1368-1644).” We can see why the glaze is called Jun, coming from the Juntai kilns in Junzhou. People use the terms Jun and Chün interchangeably, but I am going to stick with Jun through this post.
Nigel Wood adds to this, saying that although the main Jun ware-producing kilns were in the “Yu and Linru counties of Henan province,” there were several other kilns outside of Henan, too (Wood, p. 118). He estimates that “production began in the late 10th Century and may have continued into the early 15th Century” (Wood, p. 119).
WHAT’S THE MYSTERY ALL ABOUT THEN?
Jun glazes have been “the most admired and least understood of Chinese high fired effects” (Wood, p. 119). They have a “mysterious quality,” which many have guessed about over the years. Quick note: When I say “Jun effect” during the rest of this post I am using it as a shorthand for the mysterious cloudy blue opalescence we see in the best Jun glazes of the Song Dynasty.
The mystery of Jun glazes is the big question… how did they do it? What in the glaze composition causes these miraculous opalescent surfaces?
In his much-loved book Pioneer Pottery (published in 1969), Michael Cardew says, “Potters used to think that grass ash, or a high silica ash, was essential to obtaining chun opalescent glazes. But there seems to be a survival of the theory, now discarded, that opalescence was due to undissolved silica—a theory based on a supposed analogy with natural opal” (Cardew, p. 41). We know now that real opals and opalescence in glazes function differently, so this is not the best line of enquiry.
Cardew discusses other theories, too, and then concludes, “Since the theory of these colors is still uncertain, any rules on how to produce them must be purely empirical, based on evidence which is inevitably partial and incomplete.” (Cardew, p. 142). In other words, it’s anyone’s guess.
Before the 1980s, the prime suspect for creating opalescence was phosphorus. Indeed, many people today still think phosphorus is the key to creating the Jun effect. Analysis by Kingery and Vandiver dispelled this myth in their article, “Song Dynasty Jun (Chün) Ware Glazes,” published in 1983. They showed that whilst phosphorous may help, it is not the main factor contributing to the Jun effect.
In Chinese Glazes, published in 1999, Nigel Wood summarizes the guesswork to date: “During the last 60 years the extraordinary visual qualities of Jun glazes have… been attributed variously to iron phosphate, to colloidal silica, to lime phosphate or suspended carbon, and to mutually insoluble glasses” (Wood, p. 119). We needn’t go into all these theories here!
WHAT DOES THE SCIENCE SAY THEN? HOW DID THEY DO IT? DEMYSTIFY ME!
After admitting to not really knowing how Jun effects are produced, Michael Cardew says, “It is, however, generally agreed that they are optical colors; and the most likely hypothesis is that they are produced by a suspension of liquid in liquid (or rather glass in glass)[1] (Cardew, p. 142). He was on the right track here!
When Cardew talks about Juns being “optical,” he means as opposed to “pigmented.” Normally, colors in glazes are “derived simply from oxide pigments that are in suspension.” How do we know this is not the case with Jun glazes? As Wood says, “It is possible to confirm the absence of genuine blue coloring particles, or dissolved blue-coloring ions, in Jun glazes by holding a sliver of Jun glaze up to the light: the glaze appears straw-colored and its blue tone disappears entirely” (Wood, p. 120). You can see this straw color on some Jun wares where the glaze is thin, such as on the rims of bowls.
Something else is going on with Jun glazes then. The blue color is optical… in the same way that the sky looks blue to us. Okay, remind me — why does the sky look blue again?
Here is a snippet from a physics website, explaining it better than I can:
If you don’t fully get it, check out this youtube video from NASA. It’s all about the way particles in the atmosphere scatter light on the way to our eyes, making the sky seem blue. This same phenomenon is going on with the bubbles in Jun glazes.
Kingery and Vandiver explain that the creation of these bubbles it is all about liquid-liquid phase separation, “wollastonite grains have a size in the submicron and micron particle size range, giving them very effective scattering power and a white cloud effect in addition to the blue opalescence of the liquid-liquid phase without crystallization” (Kingery & Vandiver, p. 1172-3). This is written in complex language, but Tichane explains it too: “Where crystals and bubbles in glazes have the effect of making them translucent or white appearing, the true opal in Chun ware also has a coloring effect. Because of the small size of dispersed droplets (~2000A.), light is diffracted as it passed through the glaze and the result is that it appears brown in transmitted light and blue in reflected light. Since glazes are viewed in reflected light rather than transmitted, this bluish opalescence reinforces the celadon blue of a Chun glaze” (Tichane, p. 178). Glazes are not viewed in transmitted light because of the opaque clay body underneath them: Light cannot pass through the clay. This backdrop of the clay body necessarily reflects the light.
Nigel Wood agrees, saying the Jun effect is caused by “minute spherules of glass within the Jun glazes were scattering blue light” (p. 120). The size of the bubbles plays a role in determining the color of the glaze; “the ‘moon white’ qualities of slightly underfired Jun glazes are analogous to emulsions like milk, where minute droplets of fat, suspended in water, also scatter white light. In these underfired glazes the separated droplets are larger, but as firing temperatures increase, the droplets that develop in cooling are smaller, changing the glaze color from white to blue then, with more heat, to blue with a hint of purple. This ‘emulsion’ phenomenon sets Jun glazes apart from the general run of Chinese glazes” (Wood, p. 120).
Cardew surmised, “if the majority of the suspended globules are between 0.45 and 0.5µm in size, the color will be blue” (Cardew, p. 142). Wood points out, though, that it is not this simple; “the glass droplets in Jun glazes average 0.8µm and are therefore considerably finer than the wavelength of blue light (0.4-0.5µm), but through an interference effect known as Rayleigh Scattering, they supply a strong lush cast to Jun ware glazes” (Wood, p. 119-120). Thus, Jun glazes appear blue in the same way that the sky appears blue to us.
WHAT DO WE MEAN BY LIQUID-LIQUID SEPARATION EXACTLY?
Tichane lays it out in his discussion of phase separation:
“By phase separation we mean the simultaneous appearance of:
1) A gaseous phase (bubbles) in a liquid phase (glass); or
2) A liquid phase (droplets) in a liquid phase (glass); or
3) A solid phase (crystals) in a liquid phase (glass)
Common kitchen varieties of these kinds of phase separation are:
1) Meringue or whipped cream – air dispersed in a liquid
2) Mayonnaise – oil dispersed in an aqueous solution.
3) Fudge – Fine sugar crystals dispersed in a liquid.
The visual effect in both kitchen and glaze cases is to convert more-or-less transparent base liquids into translucent, opalescent, semi-opaque systems.” (Tichane, p. 173)
It is the liquid-liquid separation in the Jun glaze that accounts for the multitude of bubbles in it, which scatter the light and appear opaque and blue to us.
In his immaculately researched book, Ceramic Glazes, Parmelee says there are various systems in which liquid-liquid separation can occur. He reiterates the importance of the size of the bubbles; “The dispersed spherical particles responsible for the opacity had particle sizes in the range of 0.1 micron to 0.5 micron. Furthermore, when reheated these compositions precipitate extremely fine crystals that still further increase opacity” (Parmalee, p. 23).
Kingery and Vandiver isolate the Jun system of phase separation: “The principal constituents in Song porcelain glazes are K₂O, CaO, Al₂O3, and SiO2. Their proportions correspond to compositions in the phase tetrahedron bordered by the crystalline phases: wollastonite, anorthite, orthoclase, and cristobalite, as illustrated in Fig. 2” (Kingery & Vandiver, p.1270).
Do I fully understand this diagram? Not really. However, I find it encouraging that Kingery and Vandiver mapped these glazes so specifically. Smarter folks than I! Here’s another diagram from them; Fig. 4, below, shows the area of liquid-liquid phase separation.
We can see in this diagram that the Jun glaze compositions sit along the boundary of the region in which liquid-liquid phase separation occurs (Kingery & Vandiver, p. 1274).
This demonstrates again that Jun glazes lie in a very specific compositional zone. The silica concentration of these Song glazes is “fairly constant, falling in the range 70-73% SiO2. Along the joint between SiO2 and wollastonite, CaSiO3, the glaze can separate into two liquid phases, one rich in silica and one closer to the composition of wollastonite” (Kingery & Vandiver, p. 1270). Now we are getting somewhere. These are the liquid glasses of the liquid-liquid phase separation: one being silica rich, and the other closer to wollastonite (CaSiO3).
NOW WE KNOW WE’RE AIMING FOR THIS ZONE: WHERE THE GLAZE SPLITS. HOW DO WE ACHIEVE THIS?
Of foremost importance is the alumina:silica balance of the glaze. Kingery and Vandiver show that Jun glazes contained between “69-73% silica and just under 10% alumina,” compared to Longquan celadon glazes which contained more like “68% silica and 14.5% alumina, with the rest of the oxides being relatively comparable” (Wood, p.121).
In his recent book, Rock Glazes Unearthed, Matt Blakely corroborates this, “There are numerous factors that come in to play here but the primary requirement is balance. The flux:alumina:silica ratio must be within a rather narrow area on the glaze map that produces a glaze just on the border between clear and opaque.” He goes on, “the balance is dependent on a low alumina (around 0.3 is common) and high silica (an alumina: silica ratio of around 1:12). Alumina inhibits the effect. Too little silica and the glaze is clear, too much and it is opaque” (Blakely, p. 122).
He goes on to say he has produced Jun glazes with “alumina levels ranging from 0.32 – 0.42 but it seems to be best if the alumina comes from the feldspars in igneous rocks rather than clay” (Blakely, p. 122-3). In terms of silica, he says it is “desirable that the silica present is as fine in particle size as possible. Straw or grass ash would be useful, otherwise I suggest the silica containing rocks are milled as fine as you are able” (Blakely, p. 123). This somewhat contradicts what Tichane says about silica particle size causing variation, but we will get to a discussion of inhomogeneity later.
From my research and plugging in various recipes to Glazy, it seems that the alumina:silica ratio of Song Dynasty Jun glazes was often 1:11-1:13 or so. Blakely advises, “when using granite as the feldspar source, slightly higher is often needed to find that intermediate zone between clear and opaque.” (Blakely, p.123) This is very helpful advice, especially as I am using granite as my feldspar source.
Here is a chemical analysis of a violet Chun glaze from “Sung Sherds” (p.416) by Nils Sundius and other contributors:
What do we notice about this composition? The glaze has a silica:alumina ratio of 12.43. This is high silica, low alumina, as the literature suggests. It also has plenty of calcium, which several sources point to as important. This stands to reason if one of the liquids in the phase separation is wollastonite (requiring calcium). The titanium content is very low. It has some phosphate but not a ton. Quite a bit of potassium compared to sodium. A little magnesium. And some iron. Iron helps bring out the blue color, despite this being an optical rather than pigmented glaze.
In terms of the magnesium, Blakely has some advice: “In my own experience small amounts of magnesium are beneficial in calcium/alkali glazes. Too much and the magnesium will cause the surface to crystallize.” I have seen others, such as Cardew (p. 143), mention that a bit of talc can help encourage the Jun effect (talc is a magnesium source). Blakely continues with some advice about making Juns that are stable: “… while the Chun effect occurs in high calcium glazes, these low alumina glazes tend to be very runny so the effect can only be seen where the glaze pools in runs or on flat surfaces. To produce a relatively stable Chun glaze that can be fired to a uniform thickness, a higher alkali flux content is beneficial (around 0.25-0.3)” (Blakely, p. 122).
Kingery & Vandiver advise, “Higher silica content and additions of titania transform the iron coloration toward a more greenish hue. To obtain good Jun blues, low titania raw materials are necessary” (Kingery & Vandiver, p. 1173). Nigel Wood dedicates a whole article to this Nought-Point-Two per Cent Titanium Dioxide: A Key to Song Ceramics? in which he concludes that low titania (below 0.2%) is necessary to achieve a blue celadon.
I would also add, from my experience, that celadons and other glazes using a low percentage of iron as colorant tend to be greener in oxidation and bluer in reduction.
THE ELEPHANT IN THE ROOM: WHAT ABOUT PHOSPHORUS?
As I mentioned before, there was a time when folks thought phosphorus was the answer to making a successful Jun glaze. Whilst it is now thought of as not the answer, almost all recipes I have seen include some bone ash to provide phosphorous anyway.
It comes back to liquid-liquid phase separation: “The positive role of phosphate in these glazes is related neither to opalescence, nor to the iron coloration, but rather to its influence on bubble formation… a sample melted without phosphorous additions shows a uniform size bubble population, whereas samples melted with 1% addition of bone ash show a range of bubble sizes. Similarly, in experiments melting larger batches of material, opacity was unaffected, but bubble formation greatly increased when 1% bone ash was added to the formulation” (Kingery & Vandiver, p. 1273).
Tichane corroborates this: “Chun is really a high silica opal that is triggered and accentuated by phosphate. A lovely blue opal can be formed by adding more and more silica to a lime-feldspar glaze, but at any stage of the process the opal can be greatly identified by the addition of 0.5% phosphate” (Tichane, p. 68).
So, it does seem like phosphorus helps Jun glazes, but is not the main factor in causing them. Blakely agrees, “Although phosphorus seems to enhance the Chun effect, the flux:alumina:silica balance is more important. Glazes with P2O5 outside this balance do not exhibit the effect but, even more strikingly, glazes with no phosphorus content but just the right balance seem to spontaneously develop phase-phase separation within their glass” (Blakely, p. 122).
These is still some debate over where the phosphorus in Song Dynasty glazes came from. Tichane reckons, “The Chinese undoubtedly provided the phosphate by ash additions to the glaze slip. Many ashes contain 5% phosphate, thus a 10% addition to a glaze would provide 0.5% phosphate. I have used a 1% bone ash supplement to get the same effect and yet avoid other complicating contaminants” (Tichane, p. 69). These “complicating contaminants” in wood ash could potentially add extra interest, though. Tichane points out that it was the wood ash in Chien (tenmoku) glazes that gave hares fur effects. I will experiment using wood ash in some of my recipes to check this out.
There is some doubt over the use of wood ash in some Song Dynasty Jun glazes though. In that chemical analysis of a Jun glaze I included earlier (from Song Sherds), the manganese level was suspiciously low, as Nigel Wood points out: “MnO levels are extraordinarily low (average 0.05% MnO) – which may throw some doubt on the generally accepted view that Jun glazes contained substantial amounts of wood ash. However, almost uniquely, a wood-ash sample from a Henan Song kiln site has been shown to have contained only 0.08% MnO, so it may be that wood ashes that were particularly low in manganese oxides were available to Jun ware potters” (Wood, p. 124). It is hard to know exactly what materials these potters were using, but I would bet they primarily used a clean silica rich granite as the main ingredient, with some lime and/or wood ash as flux. Wood ash often also provides iron which helps the blue color. It seems unlikely that they were calcining and powdering bones for inclusion in their glazes. I might be wrong, though!
Another potentially interesting option could be the use of “clinker.” In Pioneer Pottery, Cardew gives his Jun recipe, which included 35% VF slag. He describes this as a “kind of ‘accidental’ frit: “In kilns fired with wood, the ash (if it contains enough silica), will, in the hottest places, melt to a clinker, not unlike the clinker formed in furnaces.” Cardew analsyed this material and found it high in calcium phosphate as well as containing some magnesium. It is possible to save this material, crush it up and use it.
WHAT ABOUT THE CLAY BODY?
As potters, we all know the influence of our clay body on our glazes. Any glaze will look different on porcelain as opposed to a dark iron bearing clay.
Cardew says, “The influence of the body or slip is very great. Because it is an optical effect and not a stain, a chun color will be deep blue over a dark slip, and still better over a dark glaze, nut oily and almost colorless over a light body” (Cardew, p. 143). Cardew footnotes here that the late T. Matsubayashi (St Ives, 1923) gave him a simple recipe for producing chun blue: use an ash glaze over tenmoku.
This was obviously Cardew’s experience, and I don’t doubt this was true for him, but I am a little skeptical. Most of the examples of Jun ware I have seen have been on pale, buff clay bodies and they were by no means colorless.
Tichane goes into good detail about the clay bodies of Song Jun wares: “The bodies of some early Chun ware are a little unusual for Sung because they are porous and sandy appearing in contrast to the porcelaneous nature of other contemporary bodies (Lung-chu’an, Yueh, Ting)… what it amounts to is that the Chun body is quite refractory and lacking in fluxes, therefore even though it is high fired, it has not become dense… analysis shows the high alumina, low potash content which would make this body quite refractory” (Tichane, p. 70).
Nigel Wood, Sotheby’s and others agree that Jun wares were generally more “thickly potted” than other wares from that period. We can also see this is true from examples of Jun wares that broke. I think the heavier walls of the pots were to allow for a thicker glaze application.
OTHER CONSIDERATIONS: GLAZE THICKNESS
Thickness is very important. You can see this on the rims of Jun bowls; where the glaze is thinner it doesn’t have the opalescence. You can see this on the underside of some Jun pots, too, where the glaze was applied less thickly.
Tichane emphasizes this, “There is one quality that distinguishes Sung ceramics from all other historical pieces – glaze thickness. If one compares Chun, Kuan, Lung-ch’uan, Chien, or Tz’u-chou to modern glazes, the one outstandingly different feature is thickness.” (Tichane, p. 159). He suggests doing a thick layer of glaze (dipped) and then spraying more on top. You can also try multiple layers, but cracking may occur.
Wood hints at why thickness is so important; “no opalescence appeared where the glazes were thin, because alumina from the clay body dissolved into the thin glaze and upset the ‘ideal’ oxide balance” (Wood, p. 124).
OTHER CONSIDERATIONS: FIRING!
The first thing to say is that temperature is important. This is kind of obvious, but with these glazes, there can be a vast difference from cone 9 to cone 11. Blakely says, “Too little heat and the Chun will be white and opaque, too much and it will be transparent” (Blakely, p. 123). Cardew agrees with this, “The colour also to some extent depends on temperature: at the lower end of the range the glaze will be opaque, with a pale-blue bloom; at the optimum temperature a stronger color develops; at still higher temperatures, only a few flecks of blue are left in an otherwise clear glaze” (Cardew p. 143). I have found this in my own experiments. Too hot and they become celadons again.
The biggest hints I think we should look for are from the historical practices of the potters in the Song Dynasty; “Jun wares were fired in both coal- and wood-burning kilns, and were set in coarse fireclay saggers, usually with one piece per sagger” (Wood, p. 119). Here is one from the collection of the Philadelphia Museum of Art:
Saggars played two main roles. They protected the pots from wood ash and encouraged a slow cooling of the pot. Thick-walled hard brick kilns also encouraged slow firings. I had wondered about how saggars affected the local atmosphere around each piece. Wood says, “Reducing gases pass easily through refractory saggars… but with more vitreous saggars, made from finer clays, perforations were sometimes necessary for good reduced effects” (Wood, p. 119).
Indirectly, this also hints at the importance of reduction to these glazes. Wood provides three Jun bowls on p. 119 SHOW THIS! fired to different temps and the difference in color of them due to this.
It is generally agreed that Jun glazes want reduction. Blakely says it is “essential” (Blakely, p. 123). Only Cardew disagrees, saying “the glaze is unaffected by variations in reduction or oxidation” (Cardew, p. 143). Much as I love Cardew, this is hogswallop. I have seen in my own experiments that the colors and Jun effects are much more pleasing in reduction.
A major difference between kilns of this area and most kilns today was that they were built much thicker and often into a hillside. Most kilns in Honan were “built partially underground or surrounded with a thick layer of local soil.” (Kingery & Vandiver, p. 1274). This extra insulation meant that these kilns took a long time to heat up and also a significantly longer time to cool down once a firing had finished.
In their experiments, Kingery and Vandiver found that opalescence was only achieved in a reduction firing that was held at 1000oC (1832oF) during the cooling. They found “a fine-scale liquid-liquid separation took place during rapid cooling… but when a sample was held at a high temperature coarsening of the structure developed, and wollastonite precipitation occurred simultaneously with an increased size of the droplets” (Kingery & Vandiver, p. 1271). That is to say, the phase separation was enhanced with a slower cooling… the “the characteristic color and opalescence of Jun glazes appear during the cooling.” This is a big clue, indicating that “the rate of cooling is an essential consideration.” (Kingery & Vandiver, p. 1272).
Nigel Wood also relays the importance of the cooling cycle: “The qualities seen in true Jun glazes, however, are not solely the result of liquid phase separation effects – they can also show a pronounced milky streakiness, sometimes combined with a white ‘sugary’ mattness, in addition to their famous opal-blue colors. These extra qualities seem to have been caused by some micro-crystallization in the glazes of the lime-silicate mineral wollastonite during cooling… The growth of wollastonite in Henan Jun glazes was encouraged by the unusually long time that Jun glazes took to cool down – a consequence of the thick walls of the Jun ware kilns, and the heavy fireclay saggars in which the Jun wares were set. South Chinese glazes (like the Wuzhou Jun’s cooled rather too fast for these lighter streaks and patches to develop, giving shinier and generally less interesting effects” (Wood, p. 123).
Even if we do not have a kiln with thick walls, saggers and extra soil insulation, we can get around this by downfiring our kilns. Whether it be wood, gas or electric, we can control the cooling cycle by adding fuel.
I will be trying various slow cooling cycles to see what works for my granite-based Jun recipes.
OTHER CONSIDERATIONS: COPPER
Copper can look absolutely stunning in combination with Jun glazes. It can turn the glaze various shades of reddish purple. Tichane investigated the use of copper in the Jun wares of the song Dynasty.
He says, “An analysis of their origin {the purplish copper areas} is complicated by the fact that copper is both mobile and fugitive in high temperature, reducing fires… Copper’s mobility can be demonstrated by painting the inside surface of a bisque bowl with copper nitrate solution. After glazing and reduction firing, the copper-red color will be found on the outside glaze surface” (Tichane, p. 71). Here Tichane shows a diagram where he painted some copper nitrate on the inside of a bowl and then it did not just appear on the inside of the bowl, but also on the outside. This was surprising to me. It actually went through the clay body.
Tichane finds that copper can volatize completely from a long-fired piece (Tichane, p. 71). Watch out for this!
He lays out a number of choices available if one wants to apply copper to pots: “One can make solutions of copper nitrate or sulphate and paint these either on the body or on the raw glaze. However, the concentration of the solution is critical, for with too little copper there is no copper at all, and with too much copper, greens result.” This sounds difficult to dial in.
He also says, “Insoluble copper compounds can also be applied. And again the concentrations are important. Copper carbonate is easier to apply that copper oxide, but either compound trends to a dark green color when applications are heavy.” Seems like it is tricky to get it just right. Even on surviving Sung pots you can tell the potters got it wrong sometimes, “their copper splashes are often either faint or discolored by green patches.” (Tichane, p. 72). Regardless of the apparent pitfalls, I think it is worth experimenting with.
A FINAL CONSIDERATION: INHOMOGONEITY
You thought we were done? No, no. Add a spoonful of inhomogeneity. Don’t panic, we are deep down the Jun ware rabbit hole.
Kingery & Vandiver analyzed many Jun glazes and concluded that some of the depth and variation in them was due to “optical inhomogeneities within the glaze.” These inhomogeneities include “liquid-liquid phase separation a few tens of nanometers in extent, anorthite crystals, wollastonite crystals, undissolved batch material, cristobalite crystals, and glaze bubbles ranging in size from the submicroscopic up to nearly a millimeter in diameter” (Kingery & Vandiver, p. 1269).
To achieve the best chun glaze results, “the presence of cord or striae arising primarily from variations in the lime content on a scale of ~0.1mm is essential.” These come from inhomogeneities in the glaze mix.
How can we do this? They suggest considering the “way in which lime is added to the glaze, the milling time, and the uniformity of mixing.” They also suggest that “local compositional variations obtained from relatively coarse ingredients are required. Excessive milling will lead to uniform and not very interesting results… insufficient mixing with local areas of substantially different compositions will give rise to local opacity… according to one sample investigated by Steger and described in Sung Sherds, when local opacity was desired, it was achieved by introducing local clumps of fine quartz particles” (Kingery & Vandiver, p. 1274).
I will be trying various levels of ball milling in my search for my own Jun glaze. One easy way to go about this would be to mill most of the glaze but then add some of the batch at the last minute and not fully sieve it. Perhaps I will try just adding some larger grains of silica, too.
Parmelee also gives a nod on this topic, “Another source of mild opacity or for variations in opacity, is that of nonhomogoneity in the fused coating… an example would be a siliceous residue from a newly dissolved quartz grain…Because PbO and BO tend to lower surface tension in glasses, cords tend to diffuse more rapidly when they are present” (Parmelee, p. 23). Therefore, we can say that higher surface tension leads to more cords. If we want more cords and inhomogeneity, steer clear of PbO and BO in our Jun formulations.
CONCLUSION
I will let Tichane lead the way here. After analyzing Jun glazes on a microscopic level, he concluded:
- “Large crystals in the layer between the glaze and body; conclusion = high firing and slow cooling
- Undissolved silica stones in the glaze show large sized sand was used in the batch, while the surrounding cristobalite crystals indicate again that slow cooling occurred.
- Small glass droplets in the glaze represent the separated glass phase that is responsible for opalization. These also reflect a slow cooling process” (Tichane, p. 78)
Tichane concludes that slow cooling is very important and that the Jun glaze is a “high fired blue celadon with a coarse refractory body. The blue glaze is due to reduced iron in a siliceous base glaze to which added phosphate gives a market opalescence… the Sung Chun glaze was probably made with wood ash and a granitic stone” (Tichane, p. 78). This all seems reasonable to me.
My main conclusions from all this research are:
o You want a high silica:alumina ratio (test from 1:11 up to 1:16)
o Some iron (around 1.5%) is necessary
o A small percentage of phosphorus seems to help: 0.5-1%
o Low titania is ideal (under 0.2% if you can)
o You want more potassium as opposed to sodium content
o Calcium should be your main flux; whether its whiting, wollastonite or wood ash
o Reduction is important
o Slow cooling is important
o Inhomogeneity may help take the Jun to the next level
o Copper can look really nice, over or under Jun glazes
o Glaze thick! Then try thicker!
o Try different clay bodies
o Get some rest
I hope this article is useful to others in the future! I will be posting my next Jun glaze experiments soon!
Works Cited
Blakely, Matthew. Rock Glazes Unearthed. Matt Blakely, 2021.
“Blue Sky.” Blue Sky and Rayleigh Scattering, http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html#c1,Accessed January 10th, 2023.
Cardew, Michael. Pioneer Pottery. Longmans, 1969.
Kingery, W. D, and P. B Vandiver. “Song Dynasty Jun (Chün) Ware Glazes.” Journal of the American Ceramic Society, vol. 62, no. 11, Nov. 1983, p. 1269–1279.
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