Raku glaze angst question please

[Lilya]

Hello everyone,

(I'm new here, my apologies if I get something wrong.)
I'm trying to replace my Gerstley Borate basic raku glaze with something less ... volatile ... and struggling. I tested alkali frit + kaolin (85:15) and alkali frit + nephelyne syenite (85:15) and found I preferred the latter; I mixed up a batch with gum solution and 1% bentonite but am having lots of problems. I've already added more water (as per the instructions on the gum) as it was super thick, but I'm still really struggling to brush it on. It's separating within literally seconds of being mixed — I'm adding more bentonite at the week-end — and bubbling like crazy when it hits any kind of texture. This includes the previous coat! I'm trying to fix the bubbles but missing a few, and I can't seem to regulate the thickness so I either end up with glaze holes over carvings and nearly bare patches on untextured bits, or glaze so thick it obscures textures, carvings, etc. On the plus side, when it works, it's lovely!

I'm reading and researching, but I can't get my head around all the different ingredients, possibilities, etc., at the moment. I know learning the chemistry is important, but I've recently had to give up my other work (literary/academic) due to cognitive issues, and this is just not sinking in. (I have autoimmune disease and autonomic dysfunction, which cause brain inflammation among lots of other fun stuff.) Ceramics had become more or less my sole income, but now it's hit and miss whether I can even glaze successfully. I'm soldiering on with learning as much as I can, but I thought I'd ask here in case anyone could please shed any light.

Is there anything I can do to fix my current recipe, or should I go with something else entirely? I have seen (in another thread here) that frit 1254 + kaolin (90:10) can work very well, so I'm thinking of trying that? (I'm in France.)

Thank you very much for any insight you might be able to offer.

Have a good whatever time of day it is for you.

 

[neilestrick]

Hi @Lilya, welcome to the forum!

To make a glaze brushable:

Mix 2 tbsp CMC and 1/4 tsp copper carbonate with 1 gallon of water. Let it sit overnight, then mix well with a submersion blender. When mixing your glaze, substitute the gum syrup for about 1/3 of the water.

I would probably toss the current glaze. Sounds like you've messed with it enough that you can't save it.

The classic 80/20 raku white crackle glaze is just a clear glaze. It was popular because Gerstley was cheap and it suspended very well. The reason it crackles is because of the thermal shock. You can get the same effect from a low fire clear glaze like a simple 90 Frit 3124 and 10 EPK. Just like the original 80/20, you need to apply it somewhat thick to get good crackle. Having some clay in the recipe will make it suspend and brush better than a frit /neph sye blend.

[Lilya]

Hi @neilestrick, thank you so much for your reply.

I can't find frit 3124 here; I've already looked and it doesn't seem to exist in Europe. I'll check again though. I know it's sold in the UK, but since Brexit, importing onto the continent is a nightmare. Is there a substitute, do you know?

For the gum, I was using a Ceradel label gum. 20g of gum:1000ml of water, left overnight. Then 850g of that solution to 1kg of dry glaze base ingredients (not including and colours), sieve, and add extra water to make the consistency like crêpe batter (France!). It's better than when I didn't use the gum at all — that was totally unbrushable — but it's definitely not great. The label doesn't specify whether it's CMC, but I've just seen that they do also sell pure CMC, albeit at a much higher price, so I'll try that next with the copper carbonate. Thank you so so much.

 

 

[neilestrick]

1 hour ago, Lilya said:

Hi @neilestrick, thank you so much for your reply.

I can't find frit 3124 here; I've already looked and it doesn't seem to exist in Europe. I'll check again though. I know it's sold in the UK, but since Brexit, importing onto the continent is a nightmare. Is there a substitute, do you know?

For the gum, I was using a Ceradel label gum. 20g of gum:1000ml of water, left overnight. Then 850g of that solution to 1kg of dry glaze base ingredients (not including and colours), sieve, and add extra water to make the consistency like crêpe batter (France!). It's better than when I didn't use the gum at all — that was totally unbrushable — but it's definitely not great. The label doesn't specify whether it's CMC, but I've just seen that they do also sell pure CMC, albeit at a much higher price, so I'll try that next with the copper carbonate. Thank you so so much.

 

 

HERE is a possible solution. Or HERE.  Definitely try the Frit 1254 you mentioned, but it's closer to Ferro 3134 than 3124, so it may not work as well.

[Lilya]

@neilestrick Brilliant, thank you heaps!

[Lilya]

Hi again @neilestrick ,

I hope you're well. No worries at all if you don't have time to answer, but I've tried the French substitution recipe on Glazy for 3124 (both versions: the one with 3134 and the one with FR8) and the results are very depressing. I made up two batches of glaze that were identical apart from the frit (3124 sub. + 10% kaolin + CMC gum solution (1/3 of water)) ... one was more brushable than the other, but both fired to a horribly rough, lumpy texture. Both look like they just haven't melted / vitrified properly. There are bumps, a few pits ... it looks grainy too. They're essentially the same even though one is 3134 and the other is FR8, so I'm thinking that maybe the substitution recipe wasn't great? (Thus far I can't find Pemco or O-Hommel here, but I'm still looking.)
Is there anything that jumps out at you as to why these glazes woudn't have worked? Again, no worries at all if not.

Many thanks again. Take care and have a good week-end.

[neilestrick]

@Lilya how were they fired?

[Bill Kielb]

4 hours ago, Lilya said:

s there anything that jumps out at you as to why these glazes woudn't have worked? Again, no worries at all if not.

I am curious if you have used any glaze calculator, even Glazy will do. Regardless of the boron source, you likely will be shooting for (UMF) 0.45 boron or greater to melt at low temperatures. This may allow you more freedom to use local boron sources to at least get the melt established.

[Lilya]

@neilestrick Thank you, raku fired to 985°C ... I usually go to 1000° with the GB / neph sy mix, but as the temp on the FR8 was stated as 850-1000, and from what I could find, the 3134 mix was 950+, I thought I'd hedge my bets. I wouldn't have been surprised if one failed, but for them both to fail in the same way ... I'm now wondering whether I mixed wrong or if it was the kaolin? I spent last night trying to compare formulae, etc., to work it out. Thank you again.

[Lilya]

@Bill Kielb Thank you for your reply. I didn't, because the 3124 substitution recipes I was using came from Glazy. So I just mixed up according to the proportions given there and then added kaolin to make the 90/10 mix that Neil mentioned in the beginning. I did get some melt — there are some shiny patches — but certainly not a consistent finish!

[Bill Kielb]

2 hours ago, Lilya said:

I didn't, because the 3124 substitution recipes I was using came from Glazy.

May I ask what was your favorite GB recipe and what Frits are available locally?

[Lilya]

@Bill Kielb Thank you again. I was using 80% GB : 20% neph sy., either plain or with added pigments / oxides. It worked really well until I got a bad batch of GB that bubbled, fizzed, pitted, cratered ... it was then I started learning about the many joys of GB and why I should change recipes asap! As I said in my original post, I struggle with the chemistry due to brain inflammation from illness (autoimmune, nervous system dysfunction, etc.), and although I am trying to grasp it all, it just isn't sinking in.

My first attempt was 85:15 alkali frit / neph sy (I also tried alk frit : kaolin, but it wasn't as nice). It was fine for dipping but terrible for brushing, it bubbled up over texture when I tried to apply it, and it separated within seconds. I tried remaking it with gum and bentonite, but I just couldn't get decent results. At least not consistently, and it was still desperate to separate. So I decided to ask here, and Neil kindly recommended the 90:10 3124 and kaolin recipe, but as I can't get 3124 in France, things got tricky!

I'm going to try a mix of 90:10 1254 and kaolin this afternoon ... 1254 is only available in France I think, but it's apparently formulated for raku and I know someone has had success with it at that ratio. I might try it also at 100% 1254, just to see what happens. Beyond that though, the most readily available frits seem to be 3110, 3134, 3195, FMTR101, 1249–1257 (Ceradel),  and FR1–12, FR413, FR644 (Solargil). There are a few other suppliers, but their stock levels seem less consistent and they're pretty much just stocking the above mentioned anyway.

Essentially, I'm looking for a transparent raku glaze that is or can be made brushable. One that looks nice on its own but that is also happy to take colours. My work tends to have texture or carvings in it as well, so something that flows relatively well when wet is essential, as otherwise I end up with black gaps over any kind of texture no matter how diligent I think I've been with a tiny brush. I guess I was spoiled with the GB.

Many thanks again to your for your participation on my glaze quest!

Ps - 1254 is formulated as follows:

Analyse Chimique : SiO2 : 30-60% / Al2O3 : 5-10% / B2O3 : 10-30% / CaO : 1-5% / BaO : 1-5% / ZnO : 1-5%
Na2O : 10-30% / K2O : 0.1-1%

Formule moléculaire :

Na2o =11.2 
Cao = 10.2 
Al2o3 =7.4 
B2o3 =27.7 
Sio2 = 43.5

Edited April 22, 2023 by Lilya

[Min]

I compared your original Gerstley Borate recipe with nepheline syenite to ones with Solargil FR8 frit and the Ceradel 1254 frit then tried to come up with something as close as possible using what you have available. Roadblock I ran into was getting the silica and alumina as low in the fritted formulas as the original Gerstley Borate recipe.

I found a blend of the two frits, FR8 and C 1254, got the boron (melter) okay but the silica and alumina are a fair bit higher. I could take the kaolin out of the recipe to lower the silica and alumina but then it will be back to sinking like a stone in the bucket so I kept it in. This means it might not melt quite as well but if you are testing glazes it might be worth a try. Other thing to try would be to fire a bit hotter if it appears not quite melted enough.

I also included what the chem looks like if you use just FR8 or C1254 at 90% with kaolin at 10%. I find the format and colours of the glaze calc program I use very jarring, these are not easy to look at. Also, what looks okay on paper doesn't always work in reality so if you try the blended version please just test a small amount first.

 

[Lilya]

@Min Oh my goodness thank you so much. I am beyond grateful, thank you thank you. And yes, the colours of the program are hideous but seeing everything there side by side helps an enormous amount, whatever colours they are. Thank you.

So having looked some things up, would I be correct in thinking (roughly) that silica and alumina kind of act the opposite to boron? I know I'm hugely oversimplifying here, but yes: boron melts, they stiffen. So looking at those, the 1254 mix is higher in Si and Al than the FR8 mix, but it's also higher in B which might make up for that? Anyway I will try all of these and see what happens, thank you so so much. My last resort was going to be ordering another batch of GB from a different supplier, praying, and then ordering loads if it works (enough to keep me going while I try more alternatives), but fingers crossed something will work here.

Honestly, thank you _/\_

Edited April 22, 2023 by Lilya
typos!

[Hulk]

"... silica and alumina kind of act the opposite to boron?"

This is a good question!

It's been almost five years since I first started reading about glazes ...I'll circle back with recommendations for reading in a day or three.

Silica is a main glass former, and the main glaze ingredient, but it doesn't melt at typical pottery kiln temperatures, hence "fluxes" are required to lower the melting point.
Boron is both a flux and a glass former. Most of us working in mid fire (cone 5/6) depend on Boron; those working at lower temperatures depend on Boron more!
Alumina is important for glasses/glazes (glaze is, essentially, glass) for toughness/strength and, as you point out, to stiffen the melt.

Here's an article on fluxes
Flux (digitalfire.com)

Edited to add ("a day or three" slipped to two weeks):

I like Susan Petersen's book The Craft and Art of Clay, particularly the glaze section, which includes an entry on each important ingredient (including colorants) and clear discussion of unity.

Edited May 6, 2023 by Hulk
recommendation for reading on glaze ingredients

[Bill Kielb]

@Lilya I believe both recipes will melt at your low temperature. Fr8 would be a visual favorite and boron greater than 0.45  (0.60) ought to melt well. Just a thought, there is room in both recipes for alumina in my view as 15:1 Si:Al is high. You may want to simply test  with a greater percentage EPK to achieve this. So 85% /15% may be worth a try to see if that works well and …… hey it’s probably more economical as well.

Just to add, you may want to stop at Sue McLeod’s site. She has a nice way of explaining glazes, many of her instruction guides are free to download and she does conduct workshops. https://suemcleodceramics.com/

Edited April 24, 2023 by Bill Kielb

[Lilya]

@Hulk Thank you very much for the explanation and the link. I realise I have a very long way to go, but I'm grateful that I'm starting to understand things a bit more. So I'll be depending on Boron quite a lot then, given my low temps!

[Lilya]

@Bill Kielb Ok, thank you so much. I'll see what happens ... I'm not using EPK (can't get it here) but another kaolin; I realise now that that in itself may be affecting things. (As an aside, I'm going to try pit firing soon partly because I want to experiment and it's my compensation for not having a wood-fired kiln, but also because it might be a nice mini break from glazes.)

Thank you lots for the Sue McLeod link. I've seen a couple of her YouTube videos actually and marked her as someone to look up but hadn't got to it yet. It's good to know that she's recommended; I'll check out her site.

Many thanks again!

[neilestrick]

13 hours ago, Lilya said:

@neilestrick Thank you, raku fired to 985°C ... I usually go to 1000° with the GB / neph sy mix, but as the temp on the FR8 was stated as 850-1000, and from what I could find, the 3134 mix was 950+, I thought I'd hedge my bets. I wouldn't have been surprised if one failed, but for them both to fail in the same way ... I'm now wondering whether I mixed wrong or if it was the kaolin? I spent last night trying to compare formulae, etc., to work it out. Thank you again.

I would not expect the kaolin to be the problem. It could be that the glaze was simply under-fired. IMO, If you're firing quickly like a typical raku firing, the best way to do it is to actually watch the melt rather than relying on a set temp, at least at first. The glaze will bubble up and then settle down and gloss over. At that point it's ready to pull. Note the temp on the pyrometer at that point and then you can fire by temp from then on. I've also found that raku glazes mature at a fairly wide range of temps since many of them are not real glazes, and that the white crackle glazes tend to like more heat than the copper patina types.

[Lilya]

@Min
Just an update (I hope you're having a lovely Sunday): I tried 1254+kaolin and FR8+kaolin this afternoon and the results are pretty good! Consistent, glossy finish on both at 1000°C ... slightly more teeny teeny tiny micropits with the FR8, but both are definitely a huge improvement over what I've seen so far with other mixes. The 1254 seems better in general and was easier to brush on, but the difference is marginal.

Neither flowed into carvings as well as the GB and I did get some black holes, but they were only over the smaller carved areas. For the bigger ones, like my roman numerals to mark the tiles, everything looks as it should. I'm going to try again (when my next batch of test tiles has been fired!) and pay special attention to filling in carved areas first. This time I just wanted to see what happened if I brushed as I would've before. I'll aslo try the 1254/FR8 blend and the 75:15 ratio that @Bill Kielb suggested.

So massive massive thanks again to you and to Bill. I'm feeling quite relieved that I'm getting somewhere! :-)

[Lilya]

@neilestrick
Thank you so much again for your help. Unfortunately, I can't watch the melt as my kiln is electric (French: it's made for bisque and raku), so I can't open it to see w/o massive heat loss. But hopefully I'm getting somewhere now ... we shall see!

Many thanks again.

[neilestrick]

38 minutes ago, Lilya said:

@neilestrick
Thank you so much again for your help. Unfortunately, I can't watch the melt as my kiln is electric (French: it's made for bisque and raku), so I can't open it to see w/o massive heat loss. But hopefully I'm getting somewhere now ... we shall see!

Many thanks again.

Does it have peepholes?

[Lilya]

@Bill Kielb I'm sorry to bother you again, but my brain has just caught up with the maths. You said "So 75% /15% may be worth a try ...", but that's only 90; am I not understanding something, or should it be 85/15 or 75/25?

Thank you so much again for your help!

[Lilya]

@neilestrick 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.

[Bill Kielb]

1 hour ago, Lilya said:

should it be 85/15 or 75/25?

Thank you so much again for your help!

Yes definite typo - sorry, from 90/10 probably through 80/20 …… so should have said 85/15. The reasoning was the Si:Al was very high mainly due to the lack of alumina so I was looking for more alumina which ought to be in your Kaolin. As far as melting at or around cone 05/04, boron in the range of 0.45 should get you there. You currently have 0.61 in one recipe and 0.70 in the other so either ought to melt well ……… even with more kaolin as it is likely this will still be greater than 0.45 boron. Additionally The Kaolin will help suspend things and ought to be more economical than the Fritt. So, 10-20% Kaolin ought to still melt, provides alumina which we appear to be short of and could help with suspension.  All probably worth a try as if the end result suits you it is likely a more economical recipe. If you don’t like the result, then no, it’s just not better. Won’t know until you try it though. I think I would test 10%, 15% and 20% Kaolin - all adding up to 100% to see if I liked one over the other.