My first kiln load with 3 macro crystal glazes

[JessHobbyPotter]

12 hours ago, oldlady said:

jess, are you wetting the pot a little while before applying the glaze?   dry bisque sucks all the moisture out of the brush before it makes a complete stroke.  quickly dunking the piece in water before you prepare the glaze, brushes, clean area is probably enough time to let it work.

Yep, I wet it... but the glaze was home made.  Next time I will be using a mix of glycerine and water to create the glaze to aid brushability.   Also, Glazenerd suggested adding some bentonite.

[JessHobbyPotter]

Update.  I made another batch of the glaze and added some glycerine, which improved the brushability.  Using the same firing schedule, I got smaller crystals (teeny!) and a lot of beautiful, delicate crazing.

Based on Glazenerd's suggestions, i tweaked the glaze recipe a bit, and this was the result (see image).

This pot was first under-fired... not on purpose, but anyhow. (I am a work in progress).  Then I added a little more glaze and re-fired it with the same schedule that I used for the first batch that was successful.  I think the larger fields of blue at the bottom did not get enough of the second coat of glaze.

Ultimately,  I want larger crystals...  I love the crazing with the crystals.  I don't love the lower part of this pot.

I am grateful (I think this weekend I ought to say thankful) for any advice from the community.

-Jess

[glazenerd]

Hi Jess:

I see you like nickel carb: rather fond of it myself. 2 things to work on-1. Over population of crystals. 2. Firing schedule; more specifically growth ramp holds. 

Zinc is used to control crystal population. In this case you need to start with an 0.50% reduction in your current recipe. EX. 25% zinc in recipe reduces to 24.5% zinc. Simply increase the silica by 0.50% to offset the zinc reduction.

Ramp hold: in between the 2 large blotches of blue is a Maltese crystal formation: some refer to it as an axe-head crystal. That formation is the biggest indication of how you need to adjust your ramp holds. In this case you need to lower your first ramp hold by 10F. EX- 1950F is lowered to 1940F.  Remember: if the recipe you are following says 2 hours @ 1975F- that could be 1950 to 1990F in your kiln. Kiln chamber size, thermocouple type, packing density, and thermal mass ( heat loss/gain) will always vary from kiln to kiln.

The large blue blotches are primarily over saturation of glaze. ( re-applied & refined) The outer crystalline ring is a low temperature growth pattern which results in a course/open threading. I can see minor grainy/course threading which is called "felting" in the crystalline world. Felting is a big indicator of excessive glaze application or excessive colorants ( cobalt is notorious for felting)

I do not want to over load you with info- address these 2 issues and go for round three. 

Tom 

[glazenerd]

As you get closer to the target growth ramp temp: crystals will close up and fill in to form round florets. Crystal formation 1 is about 20F degrees off from the "perfect" crystal formation. If you held at 1970F, you would lower to 1950F. Crystal formation 2 is about 10-15F off from the right growth temp. Watch your formations: they will tell you how to adjust the heat. 

This are "floret" crystals.