Iron Red Test Firing - 3d Series

My 2nd iron red test firing indicated a 3rd series of tests would help me better understand the variables.  My 3rd test series will explore glaze thickness and firing schedule in greater depth.

Glaze thickness has proven a sensitive variable creating substantial variations in color and tonal range. For this Bailey's Red series I created another line blend with Spanish Red additions from 8% to 14%.  I also carefully recorded the thickness of the glaze on each test using a shop-built dial indicator glaze thickness tester. This clever device is quite similar to the tool John Tilton is using to measure his crystalline glazes. I will discuss my adaptation of a dial indicator for glaze measuring chores in the next post. I will include complete information for anyone who would like to know more about my process for adapting an Ebay purchased dial indicator to measure glaze thickness.

The below test series follow suggestions by John Post to test cooling ramps from 150F /H to 125F /H  through the 1800F  > 1300F range.  Below are the results of this slower ramp with a 8% to 14% iron oxide line blend. All are fired @ cone 10 oxidation over our standard T68 stoneware.

Bailey's Red 8% > 14%—cooled at 125F /H

The above had only very slight variations in red toned results from our earlier test tiles which were cooled at 150F /H. The results were not significant enough to conclude that slowing the cooling rate improved the vibrancy of the iron red color. My most favored test (Bailey's Red 12%) had similar bright red color whether cooled at 150F /H or 125F /H.

Thickness, however, did prove to be an important factor to maximize the brilliant iron red color. I tested Bailey's Red 12% with two different glaze thickness. The below examples show a thickness of .5mm on the left and .6mm on the right, accurately measured with our dial indicator.  With a minimal increase of .1mm, the formation of the olive / black toning appears to have a significant effect. The effect of this slight difference in thickness is certainly surprising and I will do follow-up to be confirm this conclusion.
 

Bailey's Red 12% with .4mm and .5mm glaze thickness

Being able to use the dial indicator to record glaze thickness has proved to be interesting and useful. However I would not consider this tool a necessity for producing iron red glazes. Certainly potters have (for a great many years) carefully controlled their glaze thickness application with much simpler and less expensive methods. A simple needle tool can be quite effective in this respect.

The below mugs (from the same firing) illustrate varying thickness of 12% Bailey's Red from .4mm, .5mm and .6mm.  The left mug (with thinnest application) shows thin spots of dark metallic tone. The center mug produced iron red which is close to my best results and the third mug alternated the olive / black tone speckling with mingled patches of red tone.

Bailey's Red 12% with .4mm, .5mm and .6mm glaze thickness

So application of glaze does appear to be an important factor for iron red development—as long as these results are understood as a result of my test schedule, kiln equipment and glaze recipe. In other words your results may not match mine. For anyone who interested—I will give a careful explanation of purchasing, modifying and using our glaze thickness / dial indicator in the next post.

I am also going to do follow-up testing @ 100 F /H from 1800F > 1300F.  Since this is a substantial difference in cooling from the first firing (150F) and this current firing (125F)—the results could be curious and interesting.