Tuesday, February 24, 2009

On Second Thought...

As an afterthought regarding my last post, I'm thinking one of the main reasons I started this blog was to share with others how I do things as a way for us to help each other work through problems with our art. So I'll tell what I've read so far on how to do the simple, at-home test, for ceramic durability, suggested by other potters.

But 1st(to CMA) a small disclaimer:

The following information is NOT instructions and should not be interpreted as such - I, in no way will be responsible for injury to anyone due to their following of my tips, ideas, steps, or any other information regarding the way that I performed, or will perform tests, or anything else, on my pottery or art. It is the responsibility of the reader/viewer/commenter of this information to use his/her own judgment, and do his/her own research on the testing of ceramics for durability, or performing any other procedure discussed on this site prior to doing his or her own testing.

Safety Tip: I would suggest wearing a long-sleeve shirt, hand mitten potholders, and safety glasses when performing these tests.

So far, I've performed the boiling/plunging test, whereby I:

  1. Placed my liner test tiles in a zip-lock bag, placed them in the freezer for up to 24 hours (it's suggested to freeze for 24 hrs., but I don't think I froze my tiles every time for this long.

  2. Then I carefully emptied them from the bag into a large pan (I used a dutch oven/a large sauce pan with handles on the sides) with just enough boiling water to cover the tiles. Moved them around with a spoon to make sure they were all covered in the water, then removed the pan the heating element on my cook-top (electric oven) and left the tiles alone to cool for a few minutes. I was attempting to simulate, as closely as possible, the action of one pouring a boiling liquid into a cup - once the liquid is in the cup, it would stop boiling and begin cooling.

  3. I removed the tiles, using a slotted spoon, from the water and placed them on a towel to dry. After they were completely cooled, I checked each tile for crazing (small hairline cracks in the glaze which has a 'tiled' pattern to it), under a magnified light that I have in my studio. If your glaze is white like many of mine are, these cracks are sometimes very hard to see with the naked eye. See other ways for checking for craze lines below.

  4. I repeated this freezing/plunging process 2 more times for a total of 3 times, which is suggested, and so far, none of the pieces have crazed. Now through my reading up on this, I understand that crazing can still take place, anywhere from a few days to months later. In other words, there's no guarantees when performing these types of 'home tests'.

Another way I discovered by accident to check for crazing is to dab some of the water from my 'throwing bucket' (stirred a little to kick up the silica from its bottom) onto the surface of a pot or tile. If the piece is crazed, the water will evaporate or seep into the cracks leaving the silica behind in the cracks, forming a pattern which is easy to see with the naked eye. Other potters' suggestions involve using water-soluble pens (use a color that will show up on your piece) to mark the surface which will show the crazed pattern, but I don't like the idea of messing up my test tiles using any kind of ink because I'm sure, even though it's water-soluble, once it's in the cracks, it would be hard to remove, as it is sunken into the clay body, which is exposed via these cracks. Another suggestion is to use a cheap microscope for viewing. Ron Roy suggested at a workshop that Radio Shack had (note, it's been several years since I attended that workshop) one at a reasonable price.

So that's how I did my first test for crazing. I'm sure it would have been better to perform this test using cups instead of tiles, but I just wanted to get an idea of how stable these glazes are before making larger batches of them and also, I didn't have enough cups to test and was anxious to go ahead and start testing.

I do however, plan to use cups for the second test, which involves heating in the oven, then 'quenching' in room temperature water. As you can see, this must be done with caution also, as anything can happen. Talk about thermal shock! What I've read so far (again, I'd suggest that you read up on this yourself, as I could be wrong in my interpretation of this information) involves placing the cup to be tested in the oven, beginning at 250 degrs. F. and 'progressing' up to 300 degrs. F. and staying at that temp. for 30 minutes. Here is where I'm not sure how fast one should raise the temp. From what I've read, it seems it should be done slowly, but how slowly? Let the oven come up to 300 degrs. F. at it's default rate? That would be pretty fast. This inquiring mind wants to know, if you do, please leave a comment, or let me know via e-mail!

Anyway, the cup(s) is heated at 300 degrs. F. for 30 minutes, then removed (I'll wear hand mittens and safety glasses for this test!) from the oven and placed into room temperature water (it's suggested to do this step in the sink for obvious safety reasons).

This is a harsh test and one that should be done with caution, as anything can happen, which I explained in my last post. I plan to try to use my studio sink for this, as it's a deep janitorial-type fiberglass sink. First, I'll place the dutch-oven I used for my test with the tiles in the bottom of this sink, fill it with enough water to cover the cup, and give it plenty of time to reach room temp. before heating the cup (I assume this is important, as too cold (yikes!) or too warm would not give an accurate result).

I've read (NOT from the ASTM site) that the ASTM says that a piece that passes this second test 3 times should be considered stable enough for 'normal' (to some, 'normal' may be what I or others would consider 'abnormal') use. As I mentioned in my last post, I never could find on ASTM's site where they explain this test and how to do it. It looked to me like they are wanting to charge a fee for their information, but maybe I was not doing a good enough search for the ASTM C 554-93 test. Now I have read enough to know that they are NOT talking about using these pots for 'flame ware' or on the cook-top. This is the industry-level stuff you see in department stores and even most of those ceramics are not made nor intended for cook-top or even oven use. I'm also, not particularly interested in making 'ovenproof' pots, as I understand one should use a stoneware clay which is specifically formulated for ovenware use. But I guess this second test would, if my pot passed the tests, make those pieces 'ovenproof' wouldn't it? I'm just not into making casseroles, etc. at this time, but who knows, I could certainly change course at any time! I'm just mainly interested in my functional ware being microwaveable (yet another test to be performed) and have a good 'clay/glaze fit', hence the thermal shock tests. Later...


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