Techniques

  • Glazes

    An old Porcelain Stone Mine

    It’s surprising to me how often archaeological discoveries seem to be made in Jingdezhen, but then I remember that wherever I walk in this place there are deep layers of shards beneath my feet.

    A friend of mine was given samples from a recently found porcelain stone mine dating from the Five Dynasties Period.  Apparently the find has not gone unnoticed- professional antique makers have been secretly mining the site.  Luckily we have the chance to acquire some of this porcelain stone.

    I’m often dealing with unfamiliar, traditional materials of which chemical analyses are lacking or unreliable.  In these cases, I usually create a series of line blends to get a basic idea of what I’m working with.  From those first tests, one can further refine glazes using more line blends and triaxials.

    For this porcelain stone I created the following initial tests:

    • Pure porcelain stone, crushed, milled and sieved.
    • Porcelain body using porcelain stone and kaolin at 15-45%.
    • Lime-fluxed celadon glazes:
      • Porcelain stone and 10-20% Er Hui (Glaze Ash)
      • Porcelain stone and 10-20% Wollastonite
      • Porcelain stone and 10-20% Whiting

    Idealized “traditional” recipes are also based on two-component mixtures.  For glazes, porcelain stone was mixed with a flux like glaze ash.  For porcelain bodies, porcelain stone was simply mixed with a proportion of kaolin.

    Usually a single line blend of either Whiting or Wollastonite could tell you a lot about a porcelain stone.  However, porcelain stone mixed with Glaze Ash or Whiting often results in fuming/carbon trapping, so I wanted to test each flux separately.  I usually also create Dolomite or Talc tests.

    I also prepared two sets of test tiles for cone 10 and 12 firings.

    Stones of all types can be used in glazes.  Joseph Grebanier’s Chinese Stoneware Glazes lists many recipes that use locally sourced granite.  And Brian Sutherland’s Glazes from Natural Sources contains a wealth of information on the subject.

    A sledge hammer is used to break off pieces of the hard porcelain stone

    A very hard mortar and pestle is used to further break down the porcelain stone.

    The crushed porcelain stone is ball milled for four hours.

    Even after milling, the mixture needs to be sieved.

    After sitting and decanting excess water, the mixture is dried on a plaster slab.

    Because I’m in a hurry, the mixture is further dried on the stove.

    Porcelain body tests with increasing proportions of kaolin.

    Fired porcelain body tests.

    Fired porcelain glaze tests with increasing proportion of flux.

  • Jingdezhen

    Sanbao Porcelain Stone and Saggar Kiln

    Nestled in the beautiful mountains near Jingdezhen is Sanbao, a traditional source of porcelain stone. Porcelain stone comes in many types characterized by the local geography. Sanbao stone is primarily used in making porcelain bodies, but it can also be used in glazes.

    Worker removing porcelain stone from the Sanbao mine (May 2012)

    This wooden tool ensures equally sized porcelain bricks.

    Porcelain bricks are air-dried on wooden racks.

    A shrine at the mine.

    A workshop near the porcelain stone mine specializes in making kiln saggars.

  • Techniques

    Omega HH506RA Pyrometer

    In the future I’ll be adding articles to this website.  The first article that came to mind was a review of an excellent pyrometer I recently purchased, the Omega HH506RA.

    Omega HH506RA Dual Input, High Accuracy Datalogger/Thermometer ($199USD)

    I was looking for a thermometer to use primarily with my gas kiln, preferably a portable version that i could easily detach and use with other kilns in my workshop.

    My main requirements were:

    • At least two inputs for the two thermocouples (top & bottom) in my gas kiln.
    • Multiple thermocouple types.  I fire the gas kiln to cone 9-12 so prefer S-type thermocouples for their greater accuracy (0 to 1600°C continuous temperature range) and longevity.  But for my cheap bisque electric kiln I use K-type thermocouples (0 to +1100°C).
    • Real-time datalogging using a serial or USB cable which can be connected to a computer.  Ideally, the data should be transmitted in a non-propriety format which can be read directly from the port or from a simple text file.  (Logging is important as I’m also helping a ceramicist friend who wants to view firing data from his kilns while he’s out of town.)
    • Dual voltage, or preferably battery operated.
    • Rugged housing, solid build quality.

    Fluke offers the Series II Model 52 which has two inputs and datalogging.  However, from reading the manual it seems that the data can only be viewed on the computer through Fluke’s proprietary FlukeView Forms software, adding at least another hundred dollars (for the basic version) to the over $300USD cost of the thermometer.

    After looking through the Clayart mailing list archives I started looking into Omega thermometers.  Omega’s website (www.omega.com) can be quite confusing due to the abundance of options available, but after much searching it seemed that the Omega HH506RA would meet all my requirements.  A quick response from Omega support confirmed everything I needed to get things working.

    • Omega HH506RA Thermometer ($199) 2-Channel Temperature Measuring, 7 Thermocouple Types , Triple Display with Setable Backlight , Triple Display with Setable Backlight , Save Data (128 Samples with Real-Time Data), Datalogging (16 Sets, Max 1024 Data Capacity), Time, Record Interval, APO Time Setting , Software Package Included (RS232C Cable and Disk, Model HH506RA Only), °C/°F Switchable , 0.1 Resolution , Water/Splash Resistant, NEMA-4X,, Dustproof
    • Accessory HH506RA-USB-SW ($30) USB cable and software for Win98/NT/2000. The HH506RA already comes with software and a RS232 cable.  But a lot of computers don’t even have RS232 ports anymore, so I added the USB cable.
    • Miniature Thermocouple Connectors Flat Pin  (Part number SMPW-K-M for K-type, SMPW-R/S-MF for S-type.) You will need one male connector for each type of themocouple you’ll be using.  The connectors are easily be attached to the ends of thermocouple wires and then plugged into the thermometer.  I purchased two S-type connectors for the gas kiln and two K-type connectors for the electric bisque kiln.
    • R and S Type Thermocouple Extension Wire  I already had thermocouples and wire.  But if you need to purchase the wire, Omega sells it in a minimum of 25′ rolls.  Part number EXTT-RS-20-25.

    Installation

    As I mentioned, I already had thermocouples and wire.  So all I had to do was attach the thermocouple wires to the Omega connectors and then insert into the thermometer.  I was concerned that simply adding the Omega connectors and thermometer would lead to accuracy problems due to the length and gauge of the wire not being matched to the thermometer.  After many tests against my old pyrometer and cone firings, I was happy to find that the Omega is very accurate.

    As you can see in the above picture, my thermocouple wires are thick-gauge, much thicker than the Omega connectors are designed to be used with.  But they work together just fine, even though the connector covers cannot be used.  Thermocouple inputs and USB/RS232 input are all located at the top of the unit.  The display is quite clear and has an illuminated display.  Thermometer controls are fairly easy to figure out.  You need to adjust the settings to match your setup.  As you can see in the display, the current reading is 9°CS, C representing Celsius and S representing the type of thermocouple.  Mismatching the thermocouple type with thermometer settings will give incorrect readings.

    Output from the HH506RA using the Omega software

    The USB cable is easily connected to a computer.  The Windows software is quite old but simple to install and run.  The dual temperature displays and temperature difference display are very nice, but unfortunately I’ve found the graphing function unusable.  Fortunately, the simple tab-delimited text file output of the Omega software is easily imported into spreadsheet applications like Excel and Google Docs.

    Using Excel with Omega temperature output

    The Omega temperature output file as viewed in Excel and simple text (above).

    Using Google Docs

    Below, the file after being uploaded to Google Docs and viewed in a simple Line Chart.  The same could be done in Excel.
    By the way, the above graph shows my firing schedule.  This instance isn’t a particularly good firing.  Slow rise first hour to get rid of moisture, fairly fast to 800, from 800-900°C soak with slight pressure for a couple hours until kiln evens out, begin reduction just after with gradual decrease as reaching temperature, soak for another hour until Chinese cone 9 drops (about 1310°C), crash cool with full open damper until 900°C.  It’s a ten (in this case, twelve) hour firing but doesn’t take a lot of gas due to the three or more hours of relaxed soaking.

    Conclusion

    At $199USD the Omega HH506RA thermometer is an excellent value.  I’ve already used it for a year and haven’t had any problems.  It’s built solidly and the original 9v battery hasn’t died yet.  I really like the display on the computer monitor, especially the difference between the two thermocouple readings.  It would be nice if Omega had better software for looking at graphs, but since the files are easily imported into Excel this isn’t much of an issue.
  • Techniques

    New kiln

    Steel-frame fiber propane gas kiln, six venturi burners, single shelf (64x64cm).

    Steel frame of gas kiln in progress

    Steel frame of gas kiln in progress

    Steel frame of gas kiln

    Steel frame of gas kiln

    Completed gas kiln with steel frame and stainless steel panels

    Completed gas kiln with steel frame and stainless steel panels

    Inside of completed gas kiln

    Inside of completed gas kiln