Monday, July 12, 2010

Removing underglaze

One thing I worry about when blogging is that after three years, I often forget whether or not I have already posted about something. For every post I make, I usually have two or three that I meant to post, so it's hard to keep track of which ones were written and which ones I only thought about writing! Maybe there is a reason I am so fond of this "flair" from Facebook: maybe I am closer to making those "new friends" than I think!

So with apologies in case I have previously passed this tip along, I wanted to include it before I got back to using Concepts and art glazes. It's a pretty important detail, so it really deserves its own post anyway.

The post from a few days ago showed how Concepts interact with latex masking. It's an important thing because almost all underglazes are airbrushed. It's almost impossible to create an even tone with them any other way because they streak. Raw (that is, unfired) underglaze also looks opaque long before it really is, so the streaking isn't visible until after the piece has gone through it's final gloss fire. At that point, everything is permanent.

Masking is used to protect those areas that need to stay white. Because earthenware bisque is porous, it is almost impossible to completely remove color after it has been applied to the surface. The only drawback is that none of the most common forms of masking - liquid latex, wax, tape and foil - give a very precise line. They are not suited to fine detail. For that reason, the final edges are usually etched with a blade or other sharp tool.

With some underglaze colors, this works really well. Others are more resistant, or leave a more pronounced stain. For that reason, it's often a good idea to apply a barrier between the underglaze and the bisque.

I have found the best choice for this is the Duncan Cover Coat "Arctic White" (CC 101). It fires to about the same color as white earthenware, so it doesn't really effect the final color. It does etch off really easily, though.

There is more information about why lighter colors do not change darker colors in this older post.)

That's why it makes a useful mask for the last few millimeters of a pattern. The larger areas of the pattern (or in the case of my current tiles, the background) are masked with liquid latex, but the area closest to the final border is thin layer of Arctic White. Unlike the rest of the underglaze, I apply it with a small brush. It's also a little more flexible than the latex because I can decide to leave an area (ie., not etch it off) and it will fire normally. Were it latex, traditional underglaze would flake off.

A similar approach is taken with patterns that have a lot of body roaning. For situations like that, the Arctic White is airbrushed in a light coat before the other colors are added. Doing this makes the body color more fragile and prone to scuffing, but with roaned horses that is usually pretty easy to correct or at least camouflage.

Here is the Inspire tile from the previous post (the one with the bright green background). I painted the inside edge of the horse with Arctic White, then applied the liquid latex. The second tile, which isn't pictured yet, will have the horse painted first so the outside edge was painted white and the mask was applied to the background.

I'll explain the two different approaches ("background first" and "horse first") in the next post.

No comments: