how thick should your glaze be sprayed on and how do you tell if it is that thick?

[Lawrence Weathers]

I'm getting tired of the common "hose and hope" strategy.

in Mastering Cone 6 Glazes they point out that one of the most difficult things is controlling your  glaze application thickness. However, they never really discuss how to address this. Most advanced potters who talk about this claim to spray glazes by feel or intuition. Maybe after 40 years of hosing on glazes some magical intuition begins to creep into your soul, but I am far from that and am looking for a better alternative.

Yes, I see the shininess and the ripples as it gets thicker. However, when this happens seems to be dependent on a lot of things other than the thickness of the glaze, such as temperature, specific gravity of the glaze, the type of glaze, the orifice of the gun, how closure you are holding the gun, your air pressure, whether you are spring on raw bisque or over another glaze and probably 10 more variables that I can not think of off the top of my head .I've seen this pattern happen between .09 mm and 1.5 mm.

I've even been admonished by some senior potters for even trying to measure glaze thickness as opposed to learning the intuitive approach. Sure, I'd like to have that level of intuition but I'm just not sure how to get there from here.

Besides, In discussing glaze thickness with senior potters, I'm not even sure quantitatively how thick it should be. I've heard between .5 mm and 1.5 mm. At 1.5 most of my glazes are peeling off like potato chips.

I've developed a precise glaze thickness measuring tool. Using this I have been experimenting with between .20 and 1.2 mm. All I'm still not sure I know what it should be.

So my question is, how have the rest of you successful potters solved this issue?

 

[Norm Stuart]

The pre-firing thickness of a glaze coating doesn't correlate well with the fired thickness of the glaze.

I have our test tiles prepared with one, two and three dips.  By looking at the test tile you can gauge how thick you want your glaze.  Unfortunately, if I measured the thickness of the three dip area, on the top of each test tile, the thickness would be different for each glaze.

Magnesium carbonate is a low density glaze material which occupies a very large volume.  During kiln firing, most of this material is lost as CO2 leaving a very small volume of magnesium oxide.  In contrast, man-made frits are quite dense with no weight or size loss in the kiln.  So the "correct thickness" is going to be different for each glaze.

Potters refer to very plastic ball clays which flocculate well as "fat".  Dipping a tile into a glaze fat with ball clay or bentonite will create a thick coating because the clay particles have a maximum amount of space between them (filled with water molecules).  On drying this can leave a thick dry glaze coating with a high percentage of voids.  These voids fire away in the kiln leaving a far thinner glaze coating than was applied.

Similarly, measuring glaze density with a specific gravity device, or making your glaze "the thickness of cream" will create different results with each glaze.  My preference is to mix each glaze one to one with water by weight and make the test tile.  Only by referencing the test tile do I know if I prefer a one dip layer or five.

So glaze thickness and density can be useful, but only in a relative way where each glaze requires its own "correct value". 

Some commercial glazes, like Amaco, are gelled quite thickly with bentonite so the pre-fire thickness varies less from one glaze to another.

[Lawrence Weathers]

And practically, when I apply glaze, I deal with this by..... ?

[Norm Stuart]

Unless you have a previously fired test tile to refer to (with one-dip, two-dips, and three-dips) you have no realistic idea how thick to apply your glaze.

The glaze thickness you want will be affected by how you fire your kiln as well.  Compare these tiles of Ron Roy Sapphire.

With quick cooling the demarcation between three dips and one is lost.  But crystallization during very slow-cooling makes the differentiation quite clear.  So you want to customize your glaze thickness to each glaze depending on how your test tile fires.

If you ran a factory, it would be worth your time and effort to record a specific gravity for each glaze with your desired application thickness for each of your glazes - or adjust the specific gravity of each glaze so they all apply the same.  This task would require a lot of work on your part.  Are you sure that's where you're headed?

[Lawrence Weathers]

I guess what I really want to know is how big a pile of test tells you really have? I appreciate your ability to have a handy set for all questions.

I guess that is excellent background work if you're writing a sequel to MC6G.  I want to buy the first copy.

If I really understand what you're saying, unless I'm getting ready to run a factory, I'm not going to get very far beyond "hose and hope". There are just too many variables to realistically track in only one lifetime.

[Norm Stuart]

There really are a tremendous number of variables in firing ceramics and applying glazes.

I started with "Mastering Cone 6 Glazes" whilst slow-cooling at their recommended 185 degrees F per hour between 1,800 F and 1,500 F.  Trying out progressively slower slow-cools after the cone 6 firing, everyone liked how slow-cooling at 50 degrees F between 1,800 F and 1,500 F looked similar to items fired to cone 10 in a gas kiln.

With this firing variable decided I made test tiles of each glaze.  Many of the glaze recipes required adjustment with additional flux and silica to  look attractive with our very slow cool.  Some glazes like our recipe for Laguna Tang Lime simply could not be adjusted for a slow-cool - this glaze consists of high expansion crazing of the lime colored translucent glass.  Slow-cooled the glaze no longer crazes and is an opaque mustard color.

While we began making test tiles which were brown clay on one side and white clay on the other, so we could see what 1, 2 and 3 coats of glaze looked like on both light and dark clay.  But people lost interest in making these complex two-color tiles, not so much because of the time involved but because the same glaze on other white or brown clays looked different to the test tile.  With 12 different types of clay to choose from at our studio, making a test tile of each glaze on each clay would become a full-time profession.

Once you have tiles (we now typically use only a white clay) with one, two and three dips of a glaze, and you keep a lid on the glaze to maintain the ratio of water to dry ingredients - you get adept at deciding how thick to apply each glaze on your ware - but there's no substitute for additional test tiles!

When I put a lot of work into a piece, there is no substitute for making some test tiles with the clay I'm using and trying out a number of the glazes I'm thinking of using.  Some of the pieces I've made are realistic representations of food which required a large number of tests combining several shades of color.  Skin tones also require a combination of a variety of colors as well.  It's difficult painting with glaze because the pre-fire color is often different to the fired color.

This is an example of using five different Mason stain colors to replicate my skin tone.

[Lawrence Weathers]

I know flocculation also makes some difference.

You're right the safest approach is to test and test and test and test. However, there have to be some ways a staking out parameters and therefore the range of testing required. I know glazes thickness is not the total answer by any chance, but here is a link to my attempt at a glaze thickness measuring tool, which at least puts 1 dot on the graph.

Glaze depth measuring tool

There have to be some others besides an infinite number of test tiles.  Somebody please help spell them out

[Shine Chisholm]

I have found that, in determining how thick a glaze should be sprayed, three test tiles can give you a lot of information. That at least should bring you down a bit from an infinite number. I've also found that, at least for me, dips give entirely different information than sprays.

For example,Jennifer Harnetty's Cone 6 Eggshell looked like this dipped:

Pretty gross. I was sure I did something wrong.

This is what it looks like sprayed on:

The test tiles are one dip and the coat on the bowl is three layers of spray.

I do try to avoid ripples. If I get ripples, that coat is too thick.

It is funny, though, thinking about test tiles as compared to knitting swatches. You knit swatches *every* *single* project. It's not a problem or inconvenience, it's just the *only* way to know how things will work out. You can get parameters for the gauge of the yarn and thickness of the needle, but that won't tell you how you hold a needle of a particular length (which is one factor of many that affect how tightly you knit) or how much a yarn of that particular texture will slide or catch on a needle of a particular material. There's a reason nickel needles are so much more expensive than aluminum.

I was always taught to do test tiles any time any element changed. New batch of glaze? Obviously. New batch of clay? Pretty clear. Why would the mode of application be any different?

I guess the short answer to your question is that you can throw all the numbers you want at it, but the only way to actually know is to test. I've only been doing ceramics seriously for about 15 years-- and professionally only a third of that-- which is why I don't have a problem with making mistakes and trying things in different ways.

[Norm Stuart]

Test tiles are the only answer.  Listed in the Insight Database as Heidi's Eggshell, this glaze is orange where thin, or on texture breaks.  Thicker it's sort of a milky caramel.  Put it on another type of clay and it'll be different again.

Some glazes lose as much as 1/3 of their volume as gas, while other glazes lose virtually nothing. 

On the piece below I applied three layers of Ron Roy Nutmeg, roughly 2 to 3 mm thick.

I then troweled-on a 10 mm thick layer of a white crawl glaze over certain areas - slightly thinner than 1/2 inch thick. 

The crawl glaze is 50% light magnesium carbonate, a very fluffy low-density material, and 50% nepheline syenite. 

Most were convinced this 1/2 inch thick crawl glaze would run all over the kiln shelf, but the result certainly doesn't look like it resulted from a 1/2 inch thick application.  On the lower right corner of the photo you can see the remains of a thin mostly clear layer of the crawl glaze, probably excess nepheline syenite, left behind after the glaze crawled away from that area.

The correct thickness for each glaze varies dramatically, and each glaze varies even more depending on how porous the bisque is for that particular type of clay.

[Lawrence Weathers]

I think I should of stayed with welded art

[Norm Stuart]

Crawl glazes with a high percentage of Light Magnesium Carbonate and engobes are the extreme outer range of how thick glazes should be applied. 

57% of the magnesium carbonate off-gasses leaving just a small addition of magnesium oxide to the melters (in this case nepheline syenite).

Engobes are mostly clay and are fairly refractory, so they don't run.  So engobes are much like applying slip where it can be as thick as you prefer.

One big mistake people often make at our studio is applying the same thickness of glaze to paper clay bisque as they apply to other bisqued clays.  Paper-clay is another extreme example of problems with glaze thickness.

The bisqued paper clay is so porous that it absorbs a large amount of glaze without displaying much glaze powder on the surface.  So people typically apply additional layers until the glaze is as thick as they're used to seeing. 

Once the piece is at melt temperature in the kiln, the clay densifies and squeezes out the excess liquid glaze like a sponge resulting in large over-runs all over the shelf.  Thank the chemistry gods from alumina hydrate making up 50% of the kiln wash.  But this only occurs with fluid glazes. 

A very refractory glaze like "Orange Street" acts more like an engobe and refuses to run regardless of excess application.  Magnesium crawl glazes and oil spot top coat glazes are best applied on these more refractory glazes which don't run so there's less interaction with the crawl overlay or doesn't allow the oil spot layer to slide-off the ware.

As Shine Chisholm says, there's really no substitute for doing test tiles of the glaze on the specific clay you're currently using.

[Kabe Burleson]

I spray most things three slow coats and I do not know what the thickness would be. Under spraying really sucks so I always lean toward excess. If you add food coloring to the second or third coat you can tell if you have put on an even layer. Food coloring also helps when you want to create a gradient of color from say a dark blue to a lighter blue in a sky on a mural. It is impossible to see the difference in the two glazes (same glaze body with a slight variance in cobalt) but if you add some food color to the second glaze you can tell where it begins and the other ends. It helps to control the overlap. The food coloring fires out and does not effect the color of your finished glaze. Another way to keep track of how many coats you have put on is to mark the edge of your turn table and keep track of how many revolution it has made. As far as a thickness test. Maybe if while you were spraying you could stand a test tile up in the booth and spray it the same as your work and at any point you could do a scratch test on the tile and see how much was built up. Happy firing, Kabe

[Lawrence Weathers]

The food coloring is an excellent suggestion. I will also try the title trick.

I have been marking the turntable so I can count revolutions. As a sidelight, I used the turntable motor out of my blown up microwave to create a power turntable. It is really nice to have one hand free while you're spraying.

Thanks  Larry