Author archive for Derek

  • Traditional

    Traditional celadon

    Recent firing with traditional porcelain stone glaze.  In the past I’ve tried but failed to use modern materials like feldspar and kaolin to capture the beautiful, unctuous surface and depth of porcelain stone celadons.  In this glaze the coloration is completely due to iron occurring naturally in the material.

  • Techniques

    Blades

    I use X-acto blades all the time, some modified for specific tasks like carving porcelain or scraping glaze off of feet.

    I’m not sure if it’s all part of a vast X-acto conspiracy, but it seems that a lot of people don’t know that these blades can be easily & quickly sharpened?  While there are a number of sharpening methods (even just using bare fired porcelain) that will work, it can be tedious to get the sharpening angle right.  The most convenient method I have found is an angled sharpener (pictured).  Just a few quick passes through the ceramic sharpener gets the blades useable again.  It’s faster for me to sharpen the blade than switch out a dull blade for a new one.  (Unfortunately my sharpener is approximately 45% degree sharpening angle (> 20 degrees per side), it might be better to have a narrower-angled sharpener.)

    Also note that not all X-acto blades are stainless steel.  You don’t want rusty blades all over your studio or in your reclaim.  A 10 or 100-pack of stainless steel #11 blades might last you a lifetime.

     

    A step up from the X-acto blades are stainless steel surgical blades.  They come in a wide variety of shapes and sixes perfect for a number of jobs.  I usually use the blades without a handle.  They come in ten-packs and last a really long time if you sharpen them.

  • Techniques

    Pouring Glaze with a Watering Can

    I’m sure that using a garden watering can for pouring glazes is a common technique, but when I came up with the idea I thought I was a genius 🙂  The design of a watering can ensures a constant, strong stream of liquid during pouring that is perfect for glazing.  Bubbles are reduced since the watering can pours liquid from the bottom of the can.

    Adjust the specific gravity of the glaze. Here, a 250ml beaker is zeroed-out.

    The weight of 250ml glaze is 386.7. Dividing by the weight of water, 386.7/250 = 1.55. For this glaze, 1.5-1.6 is a good pouring thickness.

    Filling a watering can with glaze.

    Rotating the piece with your hand, maintain a continuous pour of glaze.

    You might need to rotate the piece backwards and forwards two or three times.

    Using a brush with watered-down glaze, fill in any holes.

    Glaze must be sufficiently watery in order to be absorbed into the hole.

    Glaze will inevitably end up on the outside of the piece. First scrape with a blade or metal rib.

    Finally, sponge off any glaze that remains on the outside.

    Pouring the Outside

    Once the inside is glazed, I will wait until the next day to glaze the outsides.  It’s important not to overload the bisque ware with water.

    I use an old electric wheel for pouring the outsides.  It’s important to rotate the wheel at sufficient speed so that glaze does not gather on the inside rim of the pot.

    Here’s a good video by John Britt about pouring the outsides on a turntable.

    Plastic strips are inserted into the splash pan to prevent glaze from spraying outwards. A heavy plaster model is tap-centered on the wheel.

    A soft piece of sponge is placed on top of the plaster to protect the inner glaze surface.

    A bowl is placed on top of the plaster model and tap-centered.

    The watering can is used to pour glaze starting from the foot. My finger stabilizes and prevents glaze from running into the foot.

    The application properties of your glaze will determine the amount of time the glaze should be poured. For this celadon glaze with an SG of 1.48 I pour from 4-5 seconds. Keep the wheel turning after glazing to ensure excess glaze is forced from the rim.

    Once the glaze has lost its glossy sheen, scrape off glaze from the bottom of the foot with a rib.

    Using a wet, clean sponge, clean off remaining glaze from foot.

    With sufficient wheel speed, the glaze should not gather too much on the inside of the pot. Once dry, excess rim glaze can be scraped off.

  • Craft

    Orton vs. Chinese Cones

    Well, that didn’t work out.  The kiln master ended up over-firing, past Chinese cone 10.  Orton cone 12 probably dropped around Chinese cone 8/9.  Looking forward to doing a better test in my own kiln.

  • Techniques

    Low-fire Electric Kiln

    I’ve finally gotten a new low-fire electric kiln.  This kiln is designed to fire up to 1000°C, so it’s useful only for on-glaze enamels and bisque.  Total cost was 2900RMB, which is about $420USD.

    The kiln uses 45cm X 50cm shelves, a common size here.

    Elements are also snaked through the bottom of the kiln, on top of which are placed bricks.

    Single K-type thermocouple placed in the middle of the kiln.

    Locking wheels, really handy.

    Control box.

    Element connectors

    Spring door.

  • Techniques

    Slow drying

    I have a couple “wet boxes”.  These are plastic bins with lids into which a layer of plaster has been poured.  The plaster is kept wet in order to maintain humidty, slowing (if not stopping) the drying process.

    However, I haven’t used the wet boxes in a long time.  I’ve found it much easier and more convenient to simply wrap each piece in it’s own plastic trash bag.  Pieces stored in this manner can be trimmed weeks or even months later.

    Ware placed on MDF bats and wrapped in plastic trash bags.

    Ware is placed mouth-down on an MDF board to prevent warping. The boards mold easily, alternatively only wrap the ware.

    After two months the ware is still wet, although the moisture is now distributed more uniformly.

    Bags of clay are double-bagged in large heavy-duty plastic bags. A piece of soaked plaster is placed in the bottom of the bag.

  • Work

    Finally Sunny

    Work usually slows to a crawl during Winter in Jingdezhen.  Although it rarely dips below freezing, the weather is very wet.  Apart from the fact that it is quite uncomfortable to work in unheated studios, clay also dries slowly.  On the rare sunny days one can find balconies filled with drying porcelain as well as heavy blankets being aired out.

  • Work

    Translucency

  • Techniques

    Kiln & Firing

    This page is in progress and will cover my kiln and firing.  For now it is just a place to store my notes.

    A typical 47kg/100lbs LPG propane tank used for firing gas kilns. Depending upon firing style, a small kiln requires 1 to 1 1/2 tanks per firing.

    The older bottles can be very dangerous. Sometimes the top pressure-release valves leak, even after closing.

    Hydraulic hoses connect the tanks to the gas line.

    In my experience, the hose connector is the most likely point of failure. Earlier models of hoses were just rubber and would start leaking at the connector after about 2 years of use. The rubber cap ring must also be regularly replaced.

    The first gauge measuring pressure directly from the tanks. Newly-filled tank pressure usually varies from anywhere between 0.2-0.6 MPa.

    Electric water heater and gas filter.

    Typical gas line connector. Inside each end is a rubber collar and plate that under pressure should eliminate leaks.

    The pressure regulator reduces the gas pressure to a level we can use in the kiln burners. I have mine set at 0.05 MPa.

    Gauge measuring outgoing pressure from regulator.

    Kiln room valves. The bottom lever valve can be used for coarse adjustment, while the top (needle?) valve is good for fine-tuning.

    The final gauge measuring pressure at the kiln. A typical firing rarely goes above 0.02MPa.

  • Work

    Bisque out, bisque in

    Almost ready to glaze

  • Work

    Old trusty (and rusty)

    I've purchased this kiln nine years ago when I first arrived in Jingdezhen. Known as a 烤花炉, this kiln is normally used for overglaze enamels and decals. I paid about $300 USD for it. Only changed two elements in nine years.

  • Work

    Cloudscape

  • Glazes

    Colors of Celadon: Iron and Titania

    Following images from Bonham’s 2014 auction, The Feng Wen Tang Collection of Early Chinese Ceramics

    A Qingbai incised conical bowl

    A Fine Yaozhou Celadon 'Peony' Carved Bowl

    The best resource I’ve found about color in Chinese glazes is Nigel Wood’s Chinese Glazes.  Chapter 8, Iron in Chinese Glazes, covers iron in detail, while celadons are covered throughout the book.

    There’s a great range of colour in Chinese celadons.  In traditional celadons, color is mostly the result of materials containing naturally-occurring iron being fired in a reduction atmosphere, with the color modified by the balance of other oxides in the glaze as well as the underlying clay body.  Relatively small amounts of titania, manganese, copper and even cobalt can affect the color in significant ways.  Ancient kilns like Yue, Hutian, Longquan, and Yaozhou became associated with certain colors and qualities of celadon, as the materials used at those kiln sites naturally contained particular blends of oxides.  In Jingdezhen, celadon glazes seem to have evolved from fairly simple geographically specific material-based recipes (e.g. 10 parts glaze stone from Yaoli, 1 part glaze ash from Leping) to extremely refined and intentional recipes with similar base glazes that incorporate additional materials for their specific ability to modify color.  This lead to glazes named not just for kiln sites but also specific colors and qualities:   天青 (sky blue with a small < .2% addition of cobalt),豆青(”bean” celadon pure green iron celadon),影青(”shadow” lake-green),粉青,玉青 (jade celadon),冬青 (winter green),鸭蛋青 (bright duck-egg green), just to name a few.

    But celadons in the Song dynasty were mostly restricted to local materials.  Thus if your local clay and glaze stone contained only trace amounts of titania and low amounts of iron (such as in towns near Jingdezhen), then you could produce the famously pure blueish-green qingbai glazes.  While if you were in the Northern Yaozhou kilns where materials naturally contained more titania, your celadons would tend towards olive-green.  (These are greatly over-simplified and generalized statements.  Regarding Yaozhou celadons, in Chinese Glazes, Chapter 6, The Stonewares of North China, Woods also mentions the blueish-grey glazes of Yaozhou, as well as the possible influence of coal firing on the color of Yaozhou celadons.)

    For me, part of the beauty of Chinese ceramics is the ability of those ancient potters to reveal the beauty of their local materials, and how the resulting aesthetic of each kiln’s wares was in large part driven by the nature of those materials.

    Of course, these days most of us make glazes by blending various “standardized” materials, with color variations resulting from additional coloring oxides.

    The following test is a simple biaxial blend showing the influence of iron and titania on the color of a base glaze.  I’m now using Pinnell Clear for all of my additive tests as I find it a much better glaze than the traditional Leach 4321.  Further tests could be done using small amounts of manganese, copper and cobalt, as well as varying the fluxes and silica:alumina ratio.  For this test I substituted Grolleg for New Zealand Halloysite, but I did not adjust the recipe to account for the slightly different chemistry.  For this test it is important that the base glaze has as little titania as possible, so it’s best not to use “dirtier” clays.  The test tiles are made from Jingdezhen “super-white” porcelain, which serves as a good blank canvas for the glaze colors.  These glazes look quite different on dirtier porcelains and stoneware.

  • Glazes

    Glazy: One Year Old

    One year old!

    Exactly one year ago, Glazy registration was opened to the public. Since then, we’ve made a ton of improvements and added many more recipes.

    Thank you!

    94% of website server fees have been paid with your generous donations. Thanks to all of you who have added recipes, photos, and contributed valuable ideas to Glazy. Special thanks to Pieter Mostert and Matt Katz for all their help.

    Notable new additions to Glazy:

     

    Stull SiO2:Al2O3 Charts

    Si:Al Charts now include a Stull overlay as well as color-coded R2O:RO Ratios. To learn more about Stull, R2O:RO ratios, and other illuminating aspects of glazes, see Matt Katz’s Introduction to Glazes Online.

     

    Extended Search

    Simply click the eyedropper icon or one of the photo swatches to search by color. Keyword search is now greatly improved with natural language text search and the ability to search for numbers.

     

    Material Safety Information

    Newly added this month are hazard warnings for each material in the recipe. There is still a lot of work to be done in Glazy to provide accurate, easily understandable safety information for potters.

     

    Todo

    There are more improvements planned. The most important change in the next few months will be the addition of material lists, including regional and supplier lists. Material lists can be shared between users and rated.

     

    WE NEED YOUR PHOTOS!

    Ceramics recipes “do not travel well” and are very sensitive to differences in materials, preparation, application, firing, and cooling. The best way to compare, critique, and refine our recipes is to share photos of our results.

    If you have photos you would like to share but find the Glazy interface too complicated, contact us and we will help. If you represent a school or studio with a lot of tests, we can help add the photos and recipes for you.

  • Photos

    Cat hotel