Post glaze raku firing for carbonation

[Gerhard]

Hi all. I'm completely new here. Also new to ceramics so please forgive my ignorance...

I have a few cups that are getting glazed this week. They are earthenware (black clay) and I'm using some very generic black and white glazes on them.

The firing will be done at my local potter shop in an electric kiln. 

Now... my question; If I do a raku firing AFTER it's been glazed, would this make the pot brittle again? 

I'm not planning on quenching or doing any crazy stuff, I simply want to smother the pots in some straw/wood shavings to blacken the (I suspect) clean white glazes coming out of the electric kiln.

Also. If the integrity of the cup will stay OK, would this make the cup unsuitable for usage? (Food safe)

Hope that makes sense

 

[Piedmont Pottery]

The postreduction would darken the unglazed clay body and any craze lines in the glaze, but for a plain white glaze I would not expect much if any chnge to the glaze.  If you were using  a metallic glaze the post reduction would definitely have an impact on the outcome.  If you just want to highlight the craze marks, I'd use India ink.  But if you want to do a raku firing, why not just do that first?

[Gerhard]

Thanks Jeff! That makes sense... and unfortunately confirms my suspicions... 

If I do the Raku process first, then afterwards fire it again with the normal glazes via electric kiln... would the end result be a sturdy properly vitrified cup... but keep the nice darkened raku effects?

 

[Piedmont Pottery]

Functional and raku are not all that compatible.  If you refire the raku pot the dark color, which id mainly carbon, would burn off in the oxidizing electric kiln. 

If you want functional ware with the white crazed raku look, I would use a well-fitting glaze on the inside and top 1" of the outside of the cup, and then use a high expansion white crackle glaze on the outside.  Fire to the maturing temperature of your clay body.  If you want the unglazed parts to look black, you could use a black clay body, or try the India ink after firing.  

[Gerhard]

Thanks Jeff, OK cool makes sense. 

I'm using a black clay already so hopefully that will turn out fine. I'm not really looking for a crackle look on the glaze, was more hoping to get the glaze to burn a bit, come out less perfect... like this picture attached... I know that's fired in a wood kiln, reduction etc... but since I don't have access to that I was hoping to find a way to cheat it a bit LOL

Anyways, really appreciate your advice. It makes perfect sense 

[Callie Beller Diesel]

It’s possible to get this look in an electric kiln without resorting to post firing shenanigans or reduction.

 

I’ve seen @Joseph Fireborn poking in every so often lately: I’m tagging him in the hopes he sees this and can advise. He makes work that looks wood fired, but comes from an electric kiln.

However, if he doesn’t see this:

 I believe you want to start with a darker clay, which you’ve got, and use a white glaze that reacts with it. If I was going to try it myself, I’d use a clay that fired to cone 6, and start with a white base glaze that had some lithium and sodium content, as those two materials can create flashing. For opacifiers, I’d start with zircopax or the South African equivalent, and maybe add some titanium if the results of zircopax alone were too flat.

Here’s a link to a glaze that would be a good starting point. https://glazy.org/recipes/185929#

[Gerhard]

Ah wow thanks Callie! That's amazing advice. 

I've been looking at some Shino glazes this morning too, most of them seems to require reduction, but I'm going to give that one a go.

One question regarding the recipe; is Nepheline Syenite food safe? 

Really excited about this

[Harold Roberts]

Nepheline Syenite is food safe. It's also higher in sodium than most feldspar. Callie is right on the money with her suggestions. I would add that a good well fitting glaze is optimal for the interior of your pots if you want them for dinnerware. Tin oxide, expensive, 3 to 5%  and rutile, cheap, around 6 to 10% are great opacifiers and enhance flashing. If you really want the raku look in an electric firing, then you might consider a crackle and or pitted glaze on the outside of the pot. People often use black ink to bring out the crackle after firing. You can even experiment with a sawdust firing after the glaze firing.

[Min]

My memory could be way off but I seem to remember @Joseph Fireborn used his modified version of Folk Art White glaze. 

[PeterH]

Eye candy on inking crackle glazes - for non-food contact surfaces.

 

[Callie Beller Diesel]

@Min I followed the link, and the first image is a piece of his, and the poster cited him. He’s got a few comments in the comments and questions section, with some important mixing tips to get the same results. 

So yes, for the OP, that would be the one to go with, lol!

I own one of his pieces, and that glaze holds up like iron.

Edited to add: 

“Syenite” is a class of rock that’s a sort of feldspar. It supplies a lot of sodium, some other fluxes and some silica and alumina. By itself it wouldn’t make a durable glaze or melt especially well, but it’s a fairly handy ingredient. It’s not good to inhale the dust, but when it’s incorporated into a recipe, as long as the resulting glaze is stable it wouldn’t contribute anything specifically toxic. Food safety and what that actually means is a whole other topic. There’s been lots written in the archives on it, but a short version is that the presence or absence of one ingredient won’t make a glaze food safe or not food safe. 

[Harold Roberts]

 I have a cone 6 matte white I use in an electric kiln that flashes when over a dark clay or a clay with a moderate amount of iron in it as you can see in the example.

This is the recipe. 

Terracotta Matte

 

Bone Ash --------------- 9

Talc --------------------- 9

EPK –------------------- 10

Flint -------------------- 10

Dolomite –---------------16

Nephelene Syenite –---- 45

 

Tin Oxide –----------–-- 4%

Red Iron Oxide --------- 1%

[Gerhard]

Thank you so much for all the amazing advice everyone! 
Harold that glaze looks amazing on those cups! Thanks for sharing. Cool I'll start gathering some raw materials and get mixing

Really excited about this