Raku glaze angst question please

[neilestrick]

3 hours ago, Lilya said:

Nope, no way to see in without opening it. It does regain heat pretty fast once it's closed again, but I'm still thinking it wouldn't be great to lower the temp so fast (through opening) and then have to re-raise? Thank you again.

Is it a top loading kiln? If so, you should be able to crack the lid just enough to see what's happening without losing much heat. Again, you'd only have to do that for one firing, to figure out what temp that glaze needs to go to.

[Min]

+1 for what Neil is saying about checking the glaze melt. It's a bit like driving a car with a blacked out windscreen otherwise, you won't know if you've reached your destination otherwise. 

 

[Lilya]

@neilestrick @Min

Thank you both. Basically it's a big cube ... it's hinged near the bottom, so the whole top lifts off — the bulk of the cube — to leave a square, er, firing platform. So I suppose if I lay on the ground level-ish with the base of the kiln (at a safe distance!) and my husband started to open it, I could see what was happening without actually letting a rush of air in. Otherwise I could just try to open and close really fast. Maybe the next time I do my test tiles, because then it won't matter too much if something goes awry.

Thanks so much again.

[Lilya]

@Bill Kielb
No worries at all. Thank you very much for the clarification and explanation. I'll make up another batch of test tiles and some mini glazes mixes of varying proportions and see what happens. More economical is definitely a good thing! My only concern / question, and this could be totally down to my ignorance, is will more alumina make the glaze stiffer / flow less well when it melts? I'm already finding that I don't have as much flow as I did with the GB; this is an issue for me given the carvings, etc., on my pieces. If more Al will reduce the flow even more, it probably won't work for me (but I will give it a try just to see). But if I'm just being dim and it doesn't work that way, then yes it could be good to have more kaolin.

Many thanks again.

[neilestrick]

15 minutes ago, Lilya said:

it's hinged near the bottom, so the whole top lifts off

Yup, I know exactly what you're talking about. If it were my kiln, I would make a peep hole in it.

[Bill Kielb]

11 minutes ago, Lilya said:

My only concern / question, and this could be totally down to my ignorance, is will more alumina make the glaze stiffer / flow less well when it melts? I'm already finding that I don't have as much flow as I did with the GB; this is an issue for me given the carvings, etc., on my pieces. If more Al will reduce the flow even more

In general, more alumina could stiffen things up. You won’t know till you try and of course increasing the temperature or time at temperature gives you some control over this as well so it even fits with the suggestion to visually confirm the glaze has fully melted. Having a 15:1 ratio - well outside what could be considered normal though can create its own issues RE: glaze melt.  With the Fritt you have, you are sort of stuck with a high starting silica level. I think likely worth the test to see.

[Lilya]

@neilestrick Ok, I'll look into that (no pun intended)!

[Lilya]

@Bill Kielb Thank you again for the explanation. I'll do some tests soon then and see what happens!

[Lilya]

Hello again kind people (and no worries if you're too busy for yet another question from me!!),

I'm still experimenting with proportions and temperatures, but in the meantime, I have another problem: the glaze I made at the week-end was much worse last night than it was first time around in terms of brushability. I made up a big bottle of CMC gum + copper carb. that I've been using to replace 1/3 of the water in the recipes, but whereas the first time I used it, it worked well enough, subsequent uses are increasingly less useable. Argh!

The first layer goes on fine, but the second layer jumps over the first when I try to brush it on. The third is even worse. And it can't stand texture. I've taken to *always* filling in textures (all my test tiles have simple faces or patterns now ha!) first with a tiny brush to try to avoid black holes, but then I'm getting depressions where the following layers of glaze won't sit nicely on the original. I've tried it thick and thin and everything in between. I try to smooth it all over in between layers and at the end, but I inevitably miss something and end up with holes, because the glaze doesn't seem to flow and fill in. Maybe I'll find a proportion that does, but I'm not there yet. Maybe I'll find a temp that works better too, but at the moment I feel like I'm shooting in the dark even when I'm looking! My last opening was at 980° ... everything was nicely melted, but I could see the holes in a piece even then. I let it go till 1000° and held it there a bit, but the holes persisted. No more appeared, but the ones that were there didn't go either. My kiln only goes to something like 1020°, so really I'm fairly limited with going much hotter. (And yes, I would love something with more functionality eventually, but that's all I could afford at the time! And to its credit, it has been absolutely brilliant and allowed me to do what I love.)

So I'm wondering if the gum has gone bad so quickly? And if I'm just chasing rainbows trying to get a non-GB, brushable raku glaze with what I can get here in France?

Thank you all so much again for your help. Whether you answer or not, I really do appreciate all the info you've given me so far.

 

Edited April 27, 2023 by Lilya

[Babs]

I am asking if your glazeis not flowing enough, too "thick"? If texture is clean, it has been wiped clean of all dust, or sluiced in a bucket of water prior to glazing AND your glaze is at right state, it "should" fill most indents. It may work bettr to fill any obvious holes after glazing. I do this when my texture has narrow deep indentation. Fettle the poblem areas then lay glaze on truoble spots.

CMC  will thicken on standing. It newds to be cooked, of course.

Second layer of glaze needs to be applied before 1st layer dries off

[Lilya]

@Babs
Thank you for your reply. The pieces are clean — no dust — and I have tried the glaze with various amounts of liquid. It seems that this mix just doesn't want to flow very well. (It is frit 1254 : kaolin, 90:10 or 85:15 so far.)

CMC needs to be cooked? The instructions I have don't mention cooking. How do you cook it please?

Thank you again.

[neilestrick]

Try using more of the CMC mix in place of water and see if that helps. Is you gum solution fairly slimy? It should be, like a syrup. You could also try adding some VeeGumT to the mix, about half as much as however much CMC you used. As Babs said, make sure the first layer isn't totally dry before adding the second. It should be dry to the touch, but no bone dry. No sure about cooking, I've never heard that term used in regards to CMC.

[Babs]

Cooking, ie powder form is mixed with  very hot water, syrup thickness and left overnight. It is usually much thicker in the a.m. and I thin again.

[PeterH]

oops, don't know how to delete.

Edited April 29, 2023 by PeterH

[Lilya]

@Babs
Ok, thank you very much! I'll try that.

@neilestrick
It wasn't very syrupy, no. Just like very slightly slimy water. I mixed with the proportions given, but maybe it wasn't enough? Veegum T would not appear to be available here.

I've tried it with first layer wet, first layer touch dry, first layer actually dry: each has its own problems. But I'll try another batch of CMC and see what happens.

Thank you both again.

[PeterH]

Can you confirm that you are using CMC, not the Ceradel label gum.

[Lilya]

@PeterH

Just CMC (+ copper carb), yes.

[Babs]

So ypu are using frit and nephsy and a little bentonite, and it is settling between brushes, i.e. you are constantly having to really mix your glaze and so the gum? From your original post. 

Am I way off to suggest super saturated solution of magnesium sulphate, Epsom salts , to help suspension? A few drops?

Maybe photos would help.

I'd be spritzing water on glaze between layers.

[Babs]

So ypu are using frit and nephsy and a little bentonite, and it is settling between brushes, i.e. you are constantly having to really mix your glaze and so the gum? From your original post. 

Am I way off to suggest super saturated saturated magnesium sulphate, Epsom salts , to help suspension?

Or glycerine if "brushability" is the prob.

What temp do you bisque to?

What temp does your clay vitrify?

Edited April 30, 2023 by Babs
Additional thoughts

[Lilya]

@Babs

Thank you for your reply. My original post was about alk frit + neph sy, but I haven't used it since then apart from on a project where I really wanted its rough, patchy look. I've been using frit 1254 + kaolin more recently ... that's what I added the CMC gum too and it's that I'm now struggling to brush. I wonder would glycerine work there?

But for the alk frit + neph sy (+ bentonite), for that I was using a pre-mixed gum blend that I added to water. The whole mix would separate literally instantly. I was mixing it in the bottle, mixing it in the dish, watching it separate on the brush, etc. I thought about what else I could add, but after my first post, Neil said that it was too far gone to save, so I sort of gave up on it. I almost threw it away actually, but I kept it (and shake it every few days ha!) because there are certain projects where I like its look as I said. Even if it is a nightmare to apply! But I had read about Epsom salts, so maybe that would be worth trying?? Or more bentonite to help with suspension?

I bisque to 980°. The clay's temps are 980 – 1300°C ... it's white at lower temps but turns grey higher.

My main problems overall are brushability  and flow, but with the alk frit + neph sy, it was the separation too. In reality, I wouldn't use the alk frit + neph sy on a regular basis anyway as it's a bit too opaque, but it would be nice to get it working better for when I do need it.

Thank you again!

[Min]

CMC can also be mixed up so it's very concentrated. This is how I have been using it lately for a glaze really high in nepheline syenite that I brush on. I get the specific gravity where I want it then use an immersion / stick blender to whiz in about 10ml of the CMC concentrate to about 500 ml of glaze. I brush some on a scrap of bisque, if the brush drags then I whiz in another 10ml and keep going until the glaze is brushable. (CMC 33.5  Water 500  pinch of copper carb) 

[Lilya]

@Min

Brilliant, thank you. I'll try that!

I've also been trying a GB : neph sy 50:50 mix with the dodgy GB I have. I was hoping that reducing the concentration might help to reduce the bubbling, etc. It's not perfect, but it is better. I do know that GB is not a forever option, but it might buy me some time to get a real replacement sorted.

Many thanks again anyway :-)

[Babs]

You could try bisquing to cone 04. Your clay will then be less porous and your glaze will brush better, water wont be sucked into the pot, along with Min's advice.

[Lilya]

@Babs

Thank you again. My kiln's max temp is 1020°, so while I could go a bit higher, I couldn't get to cone 04. I'll see what happens though, thank you.

[Babs]

Ok, butyour clay will be very porous snd hinder the glaze spplicatin. Try dunking the piece in a bucket of water then glazing, can't harm  It, let it dryif no good.