Getting Started with Crystalline Glazes

admin | Glaze Research, How-to & Studio Info | Thursday, January 31st, 2008

This is still a fairly rough draft…
I will probably be adjusting this post as I have the chance, so feel free to check back from time to time.

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I’d like to start out with a suggestion on what not to do.
When I started with crystalline glazes almost 10 years ago, I made the mistake of seeking out as many recipes as I could, then testing all of them over and over. I spent several weeks mixing and creating tests just to load a kiln, and came to terms with the fact that “shotgunning” 30+ different bases doesn’t get you anywhere as fast as you think.
The main reason is that because there were so many glazes, I couldn’t focus on the consistency of each test. I may very well have had some wonderful glazes at any one point, and never known it because of the “sloppy science” that takes over when you overwhelm yourself with too much work and information.
So, the first thing I offer is to find a proven base and stay with it. Don’t move on until you figure a firing schedule that works, then make adjustments from there.

Keep in mind that erroneous results can be achieved within any step of the process, so stay sharp!

TO BEGIN

First, it is probably a good idea to understand how to read a glaze recipe.

Second, obtain enough of the basic ingredients (frit, zinc, silica, titanium) to cover several rounds of testing, and continue glazing other pieces beyond that. Make sure that you are using the same ingredients from the same air-tight container every time.

Third, hydrate your glaze using the correct amount of water.

Fourth, establish/check the specific gravity each time you mix a new batch. You also need to be sure that you have the same water content in your glaze every time you apply it to a test, whether you mix another batch or use the remaining glaze from a successful test.

Last, always verify that the peak temperature you think your controller is hitting is accurate –Go Here for more on this. You don’t need to adjust the controller (e.g., if your using a community/school kiln) if it’s not off by much, as long as you verify the peak at each firing. To do this, place a self-supporting cone in view of the spy hole and be there when your kiln approaches your predetermined top temperature.
Actually, I strongly suggest not ever relying completely on your controller … for testing or otherwise. When it’s time to move on, get another kiln, or simply replace the controller/thermocouples, all your notes will still make sense, because you fired to a cone rather than a digital display.

Below is a glaze that does well between ^8-10. The exact cone value depends on the crystal/background ratio you want, how thick you apply the glaze, whether you have a vertical, flat, or wide shouldered form, etc.

Ferro Frit 3110 or Fusion Frit 75: 51%
Calcined Zinc Oxide: 25%
325mesh Silicon Dioxide: 24%
Add:
Titanium Dioxide: 6%
CMC: 1-2%

Another recipe without titanium
(listed on P.141 in John Britt’s book Complete Guide to High-Fire Glazes):

Ferro Frit 3110 or Fusion Frit 75: 51%
Calcined Zinc Oxide: 23%
325mesh Silicon Dioxide: 22%

Kaolin (Grolleg): 2.5
Alumina Hydrate: 1.5

When I fire tests, I use a thrown form such as this:

Crystalline Glaze Test Dome

The form is 6 inches wide at the base, offering enough surface area to give you adequate information after firing, and provides the necessary reservoir for catching glaze run-off.

1- Apply glaze consistently on each test. By this I mean paying attention to the thickness as well as where you apply thicker layers on the test. The general rule I follow is 2mm on top 1/3 of test, 1-1.5mm on the rest of the form. I prefer spraying, and since you’re only using one or two glazes, it’s pretty painless.
2- Mark a pin or needle tool, using a file (paint/ink rubs off) in 1mm increments from its tip. Use it to test how thick the glaze is applied to each piece before firing.
3- Fire to peak, using a self-supporting witness cone (^9-10).
4- Keep detailed notes concerning all of these steps. BTW: If you don’t keep notes, don’t bother reading further, and feel free to forget everything you’ve already read -because, well… that’s what will eventually happen anyway… :)

FIRING

Here is a recommended schedule:
Preheat @100F. Hold 30min - 1 hour (0.30-1.00).
Ramp 325F/hr to 2200F. Hold: 0.00
Ramp 108F/hr to 2348F. Hold until ^10 is reached.
If you use a self supporting cone, the tip should be level with the top of the triangular base.
If firing manually, shut the power off at this point and let the kiln cool as fast as possible to the first hold. If using a controller, most have the option to stop a hold using the “Skip Step” function. On my controller (Dynatrol/Bartlett V6-CF 700) this is done by hitting the keys: Review Segment-Enter-Enter. So to get my peak, I can program a hold at a slightly lower temperature for about 20 minutes. This time is meant to be excessive, allowing  a window of opportunity to watch the cone bend to the correct angle and stop the hold.
You’ll need to learn the limits of your kiln. Here is the specially designed kiln that I fire with. Some kilns don’t cool as fast as others, so there may be enough residual heat work to bend the cone past perfect. You’ll need to experiment with where you need the cone to be when you stop that stage of the firing, so that it achieves the perfect bend while the kiln is cooling.

After hitting your peak temperature, use this holding pattern to grow crystals:
Hold 1: 1985F for 2 hrs.
Hold 2: 1925 for 2 hrs.
Hold 3: 1985 for 2 hrs.
After the 3rd hold, shut the kiln off (or allow the controller to do it) and let the kiln cool naturally.

After examining the tests you can decide where to proceed. If you have too many crystals, slow the final ramp to peak. If too little, increase that rate of climb. Try this before you alter the glaze base. Understand that increasing/decreasing that rate will change the temperature you need to achieve to bend the same cone.

CREATING REFERENCES

Once you’ve established the right firing cycle, I recommend that you continue in this way:

Take the base glaze and calculate each of the three main ingredients (frit, zinc, and silica) up and down by 3-4%. Mix and fire these tests in the same specific manner you did for the original glaze. These will probably not be all that nice, but keep these tests as reference, as they will be invaluable when trying to solve problems or proceed in the future.

Lastly, try adding colorants.
Cobalt at 1-3%, Iron at 1-5%, Copper at 1-6%, Nickel at 1-3%, etc.
Don’t let this small example list or the percentage limits above keep you from exploring further. I’ll write more on this in another post later.

FINALLY

I have volumes of journal entries and firing notes leading me to the point I’m at now, and I am constantly reminded of how I’ve only scratched the surface.
As such, there is little way I can show you the limits of glaze testing beyond what is listed above, and I really have no desire to steal your own potential. A unique experience is the best way to succeed and have your work stand out.
The last thing I’ll leave you with is, have fun! If you learn to enjoy the process, your failures will have less of an impact, and your successes will become stepping stones on an amazing journey.

Crystalline Glaze Test

12 Comments »

  1. Hi,
    Its lovely 2 visit Ur site as a matter of fact since I m a studio potter as well & making crystalline pottery but I found Ur work inspirational.

    Comment by Salman — March 6, 2008 @ 8:46 pm

  2. Thanks Salman… and I found your own work to be quite nice.
    I particularly like Pot ID : 1379.

    Have fun!

    ~jesse.

    Comment by Jesse Hull — March 6, 2008 @ 8:51 pm

  3. Jesse,

    I want to begin testing glazes with increased viscosity to help minimize their flow off the work and have been considering the addition of alumina as a starting point. How do you feel about this approach? Can you suggest an addition range? Will the addition of alumina increase the refractoriness of the glaze to the point where I will need to add additional frit?

    Phil Hamling

    Comment by Phil Hamling — March 8, 2008 @ 11:26 am

  4. Adding flux (frit) and stabilizers (alumina) can produce a back & forth / tug of war effect up to a point. The reason to add alumina in many types of glaze is to create a wider window to work within at the top of your firing. This is not always the case, however –especially concerning a glaze that is melting well below the targeted peak.

    My guess is that the increase in alumina will simply encourage nucleation (resulting in more crystals than you may want to have in terms of the aesthetic), long before it impedes running enough to matter. Therefore, a focus on the specific gravity constant of your hydrated glaze(s), the peak temp (including any hold there), and application position/ thickness would seem more valuable to me. All of these things translate into how much glaze and where it is on your pot. Additions of what may become “refractory seeds”, which you’ll have to try to dissolve at a temperature when the glaze will be flowing rapidly (the variable you’re trying to minimize), may just put you into another realm of hurdles.

    Having said this, John Tilton uses additions of clay (silica/alumina) to his glaze, adding benefits to application as well as the opacity and color effects he gets (marvelous stuff!). I believe John told me that he could add 3-4% EPK to a glaze w/o too much change visually. John goes for smaller crystals, so his pots don’t get as crowded as mine would if I had to deal with the same number. But, Derek Clarkson also incorporated 1-2% alumina hydrate into his glazes to balance a formula to his liking, and he achieved huge crystals on wide open ground. -So I certainly don’t discourage testing…

    Using your glaze base, I’d just mix & fire a simple line blend, incorporating additions of alumina hydrate or grolleg kaolin at 1.5 - 3 - 4.5 - 6% for referencing where to proceed. Remember to pay attention to the variables above (specific gravity, thickness, etc), or you may not be able to reproduce any goodness that you find.

    Comment by Jesse Hull — March 8, 2008 @ 11:28 am

  5. This is a lovely site and your work is beautiful. I was interested in looking at crystalline after a (rather offensive) member of our community in NC proclaimed he was one of the best in the world. Your work and your information is so nice, so far beyond what I have seen. I have friends, Bulldog Pottery, that do some interesting crystalline work, very unusual to my untrained mind. I make jewelry with pottery stones using Jugtown Pottery glazes and have been interested in trying crystalline on them. Thank you for such a nice site.

    Comment by Jennie Keatts — August 24, 2008 @ 6:50 pm

  6. I am facinated with crystaline glazes but as an inexperienced potter I have been really nervous to start experimenting. Having read your article I’m inspired to try. Thank you for a wonderful article and site, I am constantly amazed at the generousity of ceramicists to freely share their information and knowledge

    Comment by Coral Bennett — March 26, 2009 @ 7:32 am

  7. I love your website. Planning to test my first crystal glazes soon. Unfortunately, frits and lithium carbonate are not available in my area. Can you suggest me a no-frit crystal glaze base recipe which I can test? Would appreciate if you could also suggest the firing cycle.

    Hey Nasmul,
    A good one to try is listed in Peter Ilsley’s book… I’ve found that most of the non-fritted varieties tend to become “overnucleated”.

    Comment by Nazmul — May 17, 2009 @ 10:46 pm

  8. i actually had more of a question than a comment. i’m a high school student who’s trying to make a crystalline glaze for my school. now i’m not in control of the firings or anything like that my teacher does that, is there a basic crystalline glaze recipe that you would recommend that would work without a specific heating/cooling/holding temperatures for a certain amount of time? You see my dilema? do you have any suggestions?

    Unfortunately, crystalline glazes are all about the ability to control and manipulate the firing. If you weren’t after any specific pattern, you may be able to include the glazed piece in a kiln firing to the correct peak temperature, and hope for a long cooling cycle between 2200-1950F.

    Comment by Kaitlyn — October 22, 2009 @ 7:55 pm

  9. I have enjoyed your website very much. Fantastic work.

    I am not a ceramicist. Rather I dabble in older photographic techniques and was wondering if it is possible to apply crystalline glazes to glass plates or other translucent sub-straights. I would appreciate your thoughts.

    I don’t think so. The glass plate would need to withstand (not melt or deform) the temps required for crystallines.

    Comment by Chris Anderson — November 29, 2009 @ 8:04 pm

  10. I am a Jewelry student at CSULB in a Glaze Tech class and was intrigued with the crystalline glazes and decided to create a series for my BFA show of pendants made from a porcelain that is my own recipe and crystalline glaze that is also my own. Thank you for your great information- I have a great stepping stone… I will be back to check on more posts!

    Comment by Kristi Patten — October 27, 2010 @ 9:04 pm

  11. Do you have any suggestions in mixing a large batch?
    I have issues with the results when I mix the glaze for many pieces.

    Many of the ingredients used for crystalline glazes don’t store well once hydrated. I usually mix up my base glazes in 5lb batches and dry screen them through a rotary sieve three or more times. This mixes the components uniformly and gets everything down to the same maximum particle size. I store the batch in a cool, dry, and dark environment.
    I’ll later weigh out the necessary amount for that particular piece or firing, add the correct portions of colorants, binders, water, etc, and sieve the amended wet glaze before application. The best way to get consistent results is to know how much glaze is going on the work you’re going to load in the kiln (this will come with experience). Once you get this down, you’ll rarely have any hydrated glazes left over. ~Jesse.

    Comment by Steven Kang — March 31, 2011 @ 7:37 pm

  12. Could you please clarify your cooling cycle schedule.

    Hold No2 1925 F for 2hrs.
    BUT
    Hold No3 1985 F for 2hrs.
    This is not progressive cooling.
    Should hold No3 be less than 1925 F or should the points for hold 2&3 be switched?

    Keith, it is not progressive cooling. I go up and down in temps within the crystal growing ‘window’ to get various effects.

    Comment by Keith Cherry — February 10, 2012 @ 4:51 am

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