Tuesday, May 13, 2014

Same glaze, different clays

Because we're always trying to improve the products that we make here at The Ceramic Shop, we tend to do a lot of testing. Whether we're developing the perfect wax products or finessing our glaze offerings, the more we know about our our items, the more information we can pass on to you.

One question that we get VERY frequently is a very general inquiry: 'Why doesn't the glaze on my piece look like the tile/chip/picture?' This is a question that spans glaze firing temperatures, brands, application methods...name the variable, and it has come up when we field this question.  However, in many cases, the answer is quite direct -- the base upon which you put your glaze (whether you dip, paint, or spray) will have a major effect on how your glaze turns out.

To demonstrate just how intensely your claybody can affect your final  glaze outcome, we've tested our Cone 6 Professional glaze line on four different claybodies that represent a good swath of the claybody color spectrum. We narrowed down our testing on the following claybodies:

1. Standard 365, a cone 6 grolleg porcelain.

2. Standard 240, a white cone 6 stoneware -- as white stonewares do not tend to be as crisp and bright as porcelains, we thought it would be good to show them side-by-side.

3. Standard 259, a buff  cone 4-10 stoneware -- this is a stoneware that can actually get pretty toasty in reduction, but in oxidation it remains a mid-toned tan/brown color -- and a great deal of stonewares will fall into this general color range. 

4. Standard 266, a dark brown cone 4-6 stoneware.  This is definitely on the darker end of the claybody color spectrum -- and that color is delivered by various oxides that exist in the claybody. I mention this as you peruse through our results because you will notice that the examples of glazes on this claybody in particular feature some pretty drastic results -- that is a direct result of all of the 'stuff' (er....oxides and colorants) that are in this clay, and impart their qualities to objects that come into direct chemical contact with them, such as a glaze coat.

OK!  So, onward with our test results...

Here's a good starting point -- a side-by-side comparison of all four claybodies dipped in our Classic Clear..  

This is a clear, glossy coating that really allows the underlying claybody to show through. Fairly predictable results -- basically, a layer of clear glass has been melted over the surface of these clays, allowing their natural colors to shine through. 

If you add a bit of colorant materials or opacifiers to a clear glaze, you get a translucent glaze - and here at The Ceramic Shop, we carry plenty of these. Translucent glazes are kind of like the tinted windows of the glaze world - they cast a definite tone upon the clay that they are placed on, but that clay still shines through a bit, as well.  We have had customers who are rather surprised by just how much the underlying claybody can, indeed, affect a strongly-colored, yet translucent, glaze.  Here's an example, with our Cotton Candy:

Here, some visualization can help - say you are tasked with re-painting a room with very dark walls. If you are painting that room a significantly brighter color, you might really need a primer if you want you new shade to display its full brightness. To this end, I often suggest to people who work with darker claybodies to consider using a light-colored or white slip over the areas they might like to glaze in a bright color. And, sure, this sort of analogy makes sense when you think about the nature of translucent glazes. 

However, we also frequently have clients express their surprise at just how significantly claybody color affects opaque glazes. These are glazes that are completely solid and do not allow light to pass through; as such, it makes sense that you might think of them as less influenced by the color of the surface that they cover, right? The thing is, though, many reactions you see in a glaze surface are distinct chemical reactions that rely on the interaction of different materials -- and, for color response, this often means oxides. Red iron oxide, for example, is a very common -- and very powerful! -- material that can be found in many, many different clays and glazes. The dark stoneware that we used for these tests very likely contains this, for instance. 

When a glazed vessel goes through the firing process, it's not simply a matter of the glaze becoming fused to the clay's surface. A very small zone of mixing actually happens, where the interface between the two is a clay-glaze hybrid. This means that any oxides that the claybody might contain can indeed actually become physically enmeshed into the glaze -- and that, in turn, can alter the end-result color, even when the glaze is opaque. Here's an example with our very popular Tidal Pool glaze:

As you can see, this opaque blue-green glaze looks pretty much the same on whiter and lighter clays - but it looks completely different on the dark stoneware. The colorants that form the green/blue tones in the glaze have clearly been overpowered by the coloring oxides in the dark stoneware's claybody, resulting in a brown -- NOT green -- glaze. While we expected a degree of variation, this was one of those tests where the outcome was actually a bit more drastic than we thought it would be, which just goes to show the importance of testing your glazes whenever possible!

For a final example, I want to demonstrate just how much an underlying claybody can affect the look of an opaque, but lighter-colored, glaze. Most studios are outfitted with some sort of an opaque white glaze, and this tends to be a look that I use as my go-to. Here at The Ceramic Shop, we have a White Gloss and a White Matte, and both are unsurprisingly popular. Below, though, check out just how much a darker claybody can affect an opaque white glaze:

The first three examples are fairly predictable, but it's clear that the dark stoneware delivers significant effect to this particular glaze. For the record, these tiles were all fired in the same firing and photographed in the same light. I mention this because the change in the white matte when applied to a darker claybody is, indeed, remarkable. 

If you use or are thinking about using our Cone 6 professional glazes, you'll be happy to know we have been working hard to compile user resources on our website -- and we have many more tile comparison charts, featuring your favorite Ceramic Shop glazes. We will be adding to this until it's done! You can check out our application and firing suggestions here.

If you have used The Ceramic Shop's Cone 6 Professional Glazes and have some tips, tricks, or results you would like to share, we'd love to hear from you! We welcome comments here on the blog and we love to post customer work on our Facebook page.

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