Author archive for Derek

  • 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

  • Techniques

    Simple plaster drop-out molds on the wheel

  • Techniques

    Razor trimming

    I’m not sure if double-edged safety razor are still available in the West, but here in Jingdezhen they are an essential trimming tool.  These razors are thin, sharp, and most importantly flexible.  Great for wheel-trimming details on small forms, or for scraping hand-built objects.  The most used brand is Flying Eagle.  I get the more expensive stainless steel ones.  At 5RMB for a 5-pack, each blade is about 15¢ USD.

    Two brands of safety razors available in China

    Using a Dremel or similar tool, edges of both thin and thick razors can be ground for specific uses, like scraping glaze off these plate feet.

    A Dremel tool was used to create a notch in this blade for use in scraping glaze off this unique foot.

  • Glazes

    Glaze Transparency Test

    Recently I’ve been wondering if there’s a reliable way to test glazes for transparency.  A method that would allow one to compare results from different firings and glaze types.

    Paint manufacturers have a system for testing paint opacity that uses a black and white card from which a contrast ratio can be calculated. The primary manufacturer is Leneta.

    I couldn’t find any parallels in the ceramics industry.

    I wanted to try a similar method using porcelain (white) and stain/colored porcelain (black), adjusting the results to account for the fact that our whites and blacks are not pure.

    Paint Opacity Chart from Leneta

    Using my whitest porcelain, I created a colored slip adding 8% of a local black stain.  (Ideally one would use a standard mason stain.)  Adding Darvan, I made a thin slipcast slab that I then cut into small squares.

    Cutting square slabs of stained porcelain

    Using the same casting porcelain I made a thicker slipcast slab which was then cut into square test tiles.  The black stained squares were then applied to each test tile and rubbed flat.  Finally, the tiles were bisque fired in the hopes of minimizing contamination of the glaze when dipping.

    Test tiles after adding black-stained squares.

    Two 100 gram batches of glaze were prepared:  Pinnell Clear and Pinnell Clear with added 10% Zircopax.  Using volumetric blending I created tests in 2% increments.  The tiles were dipped in the test glazes.  Ideally, steps would be taken to ensure even thicknesses of glaze.

    Test tiles after firing in reduction to Orton cone 10.

    The fired tests display a nice opacity gradation as zircopax is added to the glaze.

    Unsure of the best way to measure transparency (or opacity) using these test tiles, I tried the simplest approach I could think of.  Adjusting the image to greyscale, I averaged the colors of the white test tiles as well as each black-stained square.  Below are the Brightness levels measured in Photoshop using the HSB scale.  If these tests were going to be made consistent across firings, I suppose one could normalize the photos based on the color of the unfired white porcelain body.

    For the opacifying power of Zircopax relative to this specific test, I created an opacity scale in Photoshop using the 0% glaze as a baseline and then matched the tests to this scale.  According to the scale, a 4% addition of Zircopax opacifies the glaze by 30%, while a 10% addition of Zircopax opacifies the glaze by 70%.  I’m probably vastly over-simplifying things.  For instance, I didn’t take into account the fact that the entire test whitens as Zircopax is added.  Also, there will probably be few times in ceramics where there is a neat linear relationship, for instance adding 14% Zircopax to the glaze won’t necessarily get me to 100% opacity.

    Below is a closeup of the black squares.  If I had made these tiles more consistently, with a crisp, straight border between the black and white porcelain, it might also be possible to compare diffusion.

    Close-up of colored squares.

  • Antiques

    Wanli Pheasant

    Blue & White pheasant head on a Wanli dish.

    Single photo from iPhone with Eyeskey lens.  More details here.

  • Glazes

    Jun

    A Jun glaze on stoneware from the last kiln

  • Uncategorized

    Sashi-ire hana no futami 挿入花の二見

    Images of Japanese flower arrangement from the woodblock-printed book Sashi-ire hana no futami (挿入花の二見) made in 1798 by the amazing Katsushika Hokusai.

    Images generously made available by the British Museum.  I have cropped and adjusted levels of the original scans.  These are full-sized images, right-click to save.

    Two beautiful books of modern Japanese flower arrangement by Kawase Toshiro:

    The Records of Flowers in Four Seasons

    One Day One Flower

  • Uncategorized

    Kawase Hasui

    A wonderfully romantic view of a Japanese pottery.  Link here.  Ukiyo-e.org is an amazing collection of Japanese woodblock prints.

    Kawase Hasui

  • Craft

    Brother Thomas Bezanson

    Brother Thomas Bezanson

    From Creations in Clay, a book of essays and photographs of Brother Thomas Bezanson’s work:

    For many years of my work as a potter I was concerned with learning the skills and technologies proper to the potter’s art.  I was focused on looking back to accomplishments of the past; I was open to those men and women who shaped the tradition of ceramic art.  They were my teachers, by necessity of my ignorance.

    Then at some point in a time out of time they left me standing, so to speak, in a dark forest where their ability to guide me came to an end; there was no longer a path of the past to follow.  There were no paths at all, except the one I was called to make for myself if I did not want to be just another derivative, condemned to repeat the past, or bootleg from the present.

    At this juncture I gradually became aware that my new teacher and best guide was my own work itself.  It led me into myself, into my own inner experience.

    Porcelain vase with copper red glaze

    Wintermoon (Vase)

    Vase

    Hexagonal platter

    Jar with cover

    Tea bowl

  • Photos

    Scanning Test Tiles

    Having purchased a scanner for digitizing my family’s old photos, I had the brilliant idea to also scan glaze test tiles.  I thought I was a genius until Matthew Katz mentioned that he had been scanning tiles for the past ten years.

    Matthew noted that CCD scanners have a greater depth of field, which is great for three-dimensional objects like test tiles.  Because of his recommendation I purchased the Canon 9000F Mark II.

    I’m not a scanner expert and have never calibrated a scanner before.  I already have an X-Rite ColorChecker Classic for photography, and this color card can be used with X-Rite’s i1Profiler (i1Publish) software to create a scanner profile.  Unfortunately, the software license seems to be very expensive.

    I tried Argyll CMS (http://www.argyllcms.com/) but results using the generated ICC profile were worse than the default output.

    Here’s a scan of some test tiles.  I had to adjust the Exposure in Photoshop by about +1 stop.  Notice the reflections on some test tiles that were not flat.

     

    2400dpi scan, 1.3GB TIFF

    Enable large image scans on Canon 9000F Mark II

    The Canon software is really frustrating- by default it wouldn’t let me scan a file greater than a set limit (10208 x 14032 pixels, or larger than 100MB).  I finally found a solution hidden away in the software settings.

    By default, ScanGear won't let you scan images greater than a seemingly arbitrarily set limit.

    1. Open IJ Scan Utility and click Settings

    2. Select ScanGear and check Enable large image scans

    3. ScanGear will now scan files over 100MB. However, you must manually enter dpi greater than 1200, such as 2400, 4800, 9600.

    Comparison with DSLR

    I have a relatively old and cheap Canon EOS Rebel T2i with a 18MP sensor.  In comparison with the Canon 9000F scans, the photos from my camera are smaller.  However, they seem to contain just as much if not more detail and better colors.  If needed I can adjust lighting conditions and camera settings to reduce reflections and adjust exposure.  On the other hand, the scan had some reflections that I could not eliminate.

    It also takes less time for me to take photos than scan at 2400dpi.

    Below are comparisons of the scan and the photos.  In particular, the dark glazes came out very poorly on the scanner.

    Tianmu glaze tile. Canon 9000f Mark II 2400dpi, adjust exposure +1.

    Tianmu glaze tile. Natural light, 18MP EOS Rebel T2i photo

    Ash glaze tile. Canon 9000f Mark II 2400dpi, adjust exposure +1.

    Ash glaze tile. Natural light, 18MP EOS Rebel T2i photo

    Detail of Canon 9000F Mark II scan

    Detail of 18MP EOS Rebel T2i photo

    Conclusion

    In conclusion, while the Canon 9000F is great for scanning old photos and documents, I still haven’t found a way to scan glaze tiles that beats results from my old DSLR.