Confusing Glaze Result

[Joseph Fireborn]

So I have been working with a glaze for a long time and I stopped using it for a while and I have since spent some time going back to it. I did a test run and fired it and it came out a completely different color, a pastey yellow.  So I thought okay, maybe I just did something wrong. I mixed another batch up and fired it side by side with several other tests just to be certain.

The clay body is Red Rock covered with a black slip. In the past, this has came out a nice grey.

My other test was Standard 365 and HW Brownstone, both also covered in the same black slip. Everything was fired on the same shelf, right beside each other, glaze batch was applied in the same exact manner, and the slip batch was all the same. Fired with cones to verify temps as well.

Again the red rock base with the black slip over came out pastey yellow. The brownstone and the porcelain came out as the redrock used to come out, a light greyish hue. Now I am really confused. I am not doing anything different between the application and in the past, the red rock has come out exactly the same as the other two bodies do. I have a yunomi in my gallery of this exact result. So I am guessing that Highwater must have sourced a new material supplier or the old one ran out?  All I know is it is now directly affecting my glaze? I can't be for certain, but I can't think of any other logical reasons for this sudden change. At first, I thought it was something I was doing, but after this test, I am confident that it isn't anything I am doing as the other bodies produce the exact result that the red rock used to produce. 

Does anyone have any other ideas? I guess I am just going to finish using my red rock and just finally convert to using 100% porcelain all the time. I don't want to throw highwater under the bus here as I like their clays a lot, but I am tired of the quality control issues I keep having with them. I get bags that are too hard to use and bags that I have to dry out. I really enjoyed their clay bodies in the past, but I feel like something is wrong somewhere. I understand that the bags won't always be the same, but still what in the world is going on when glazes are completely coming out differently than a few months ago. I bought these boxes of red rock when I started potting again this spring, so it was different box and batch from the ones where I got the grey result. 

[Magnolia Mud Research]

Joseph,

Have you fired the subject clay body without the slip and glaze, that is fired it raw, and compared it with a fired raw piece from the older bag of the subject clay body? 
 
LT

 

[glazenerd]

Joseph:

These are all stoneware bodies with various levels of iron in each. All are cone six, all with clear glaze: so the color is coming from the clay blends only. Obviously, the dark brown on the lower right has the highest content, and the blush color the lowest. The deeper blush has iron, with natural magnesium. The yellow/ straw is the middle of the road iron levels. I have a grey floating around somewhere. So yes, the possibility of lower iron in the raw clay is a possibility. I think LT's suggestion will help narrow down the cause.

 

[GEP]

@Joseph Fireborn fwiw, I have some brand new boxes of Red Rock that I got in mid March. It appears to be exactly the same as before.

[Joseph Fireborn]

30 minutes ago, GEP said:

@Joseph Fireborn fwiw, I have some brand new boxes of Red Rock that I got in mid March. It appears to be exactly the same as before.

That is good info. I wonder if I just got a bad batch or I'm crazy. Both are possible. I really can't explain or figure out why all of the sudden something I did many many times I can't get on the base body but I can on every other body.

[Joseph Fireborn]

2 hours ago, Magnolia Mud Research said:

Joseph,

Have you fired the subject clay body without the slip and glaze, that is fired it raw, and compared it with a fired raw piece from the older bag of the subject clay body? 
 
LT

 

I haven't done this! I will try it soon.

[Joseph Fireborn]

Just some pictures for comparison.

Old Red Rock Results:

A cup I kept(left) vs the cup(right) that I just pulled out today. Same techniques as before:

Porcelain base: 

I mean I am not unhappy with the result of the porcelain. It is rather beautiful itself, I just am so confused at how it can be so darn different.

Anyways, I guess it doesn't really matter. I am firing an old redrock pot and a new redrock pot right now, so we can look at that result tomorrow sometime, although I don't expect to see any difference. I think one of the chemicals in the clay was changed or something, I don't mean as in a new formulation, but as they changed suppliers of a chemical, which I mean happens if you get a better deal. But I think whatever that chemical is, has a drastic effect on this glaze.

Thanks for all your ideas and thoughts. It really isn't worth worrying about anymore. I think it just sums up my decision to go back to porcelain and stay there. I enjoy throwing it more than stoneware anyways, and I have less glaze issues single firing it over stoneware(occasional bloating, or pinholes). So c'est la vie stoneware!

[glazenerd]

Joseph:

The recent complaints I have seen about clay being too wet, or not acting or firing like before; is not really manufacturer related. Like the big Custer change back in 1999; some of the raw clay mines are older; and are digging into new areas. Some of the old tried and true ball clays have enough compositional changes, that in turn effect water retention, color, etc. I would suspect clay makers are working through the changes; some will require a fair amount of time to resolve.

i would recommend you stay away from any porcelain labeled as " translucent." Given the glazes you are using and the application rates: they would cause you some aggravating pin hole issues. Remember, higher translucency = higher flux levels! which = higher off gassing of spars, which = higher incidence of pin holes. Just as you are finding out that dark and red stoneware bodies can have variations in natural iron and magnesium levels; which effect glaze color. So again, raw clay materials are changing as they dig through the strata.

There is an old tried and true ball clay that has been used in Terra Sig for years. I recently received an updated analysis showing higher carbon, sulfur; and larger particle size distributions. I tested the WOPL index last year: it now holds ten percent more water than it did five years ago. It is widely used in stoneware as well, which would translate to "wet" clay. It also means Terra Sig will not settle out like before, and the SG will be different. the higher CEC and WOPL changes the physical properties of the end product.

* notice I intentionally left out producers and product names. 

Tom