1st Porcelain Glaze Firing

[SweetheartSister]

I have a few dozen porcelain pendants in the kiln right now. This is my first time using porcelain and since having a bad experience with crazing on my last pendants (in earthenware) I really don't want to mess it up. This is what I've done so far -

1. Bisque fire to Cone 05 at medium speed. Although I have now discovered that if you don't put a hold on my electric kiln, it will shut off when it reaches each temperature and will not soak the ware at that temperature. So going by my pyrometric cones, they reached cone 06 (orton cone puddled) but did not quite bend my 05 cone.

 

2. Then I hand painted my pieces with Amaco Velvet underglazes and set the electric kiln to cone 7, firing at medium speed once again. This means it will definitely mature at cone 6. Firing range of the slip I'm using is 1220 - 1300 celcius. Cone 6 is 1220. I've put orton cones in again to be sure that the ware reaches cone 6 and therefore vitrifies.

 

3. The final stage will be to apply the transparent overglaze. This also has a firing range of 1220 - 1300. Should I fire at Cone 7 again? Should I fire on the fast setting or on the medium again? I have a Paragon Firefly kiln and according to the manual the medium speed is 'standard', the fast speed is 20% faster and the slow is 20% slower.

 

I want to make sure I get these right this time, but feel quite nervous. These porcelain pendants will be used as part of fine jewellery pieces and I want them to last many many years and not craze on the surface.

 

Thanks so much everyone.

[liambesaw]

Next time apply the underglaze before you bisque @ leather hard.  That way you only have to fire twice.  Underglaze is usually applied to greenware and bisqued.

[Min]

@SweetheartSister, the main reason glazes craze is because they don't fit the claybody.   Crazing happens because the coefficient of expansion of the the glaze and clay are too far apart. As the glaze cools if it contracts more than the clay it will be in too much tension and it will craze. The only way to know if the glaze is going to be craze free is to do a test piece or two.

Since you have already fired the pieces to maturity you could use a lowfire glaze on them now, test before you do your real pieces, and glaze fire to the recommended glaze temperature/cone.

Or you could use ^6 glazes and refire to maturity again. There is a risk of the clay bloating if it gets overfired. As you have already fired these pieces to maturity once the added heatwork involved in firing to top temp again increases the risk of bloating (or warping and slumping). Tiny kilns like yours can fire super fast, I would use the slow speed the first time and see how the glazes turn out on test pieces.  The glaze isn't going to want to stick to the now mature clay, brushing glazes contain gums to help them adhere and flow smoothly, if you have the option of using a brushing glaze that would go on easier than a dipping glaze. Are these the same pieces that you were asking about stilting when firing?

If you want to apply the underglazes to bisqued clay then you could either glaze them after the underglaze has thoroughly dried on your 05 bisque or rebisque them to a low temperature to burn off the gums etc that they contain. Don't need to go very hot, just to around 820C (approx cone 015). 

[SweetheartSister]

13 hours ago, Min said:

@SweetheartSister, the main reason glazes craze is because they don't fit the claybody.   Crazing happens because the coefficient of expansion of the the glaze and clay are too far apart. As the glaze cools if it contracts more than the clay it will be in too much tension and it will craze. The only way to know if the glaze is going to be craze free is to do a test piece or two.

Since you have already fired the pieces to maturity you could use a lowfire glaze on them now, test before you do your real pieces, and glaze fire to the recommended glaze temperature/cone.

Or you could use ^6 glazes and refire to maturity again. There is a risk of the clay bloating if it gets overfired. As you have already fired these pieces to maturity once the added heatwork involved in firing to top temp again increases the risk of bloating (or warping and slumping). Tiny kilns like yours can fire super fast, I would use the slow speed the first time and see how the glazes turn out on test pieces.  The glaze isn't going to want to stick to the now mature clay, brushing glazes contain gums to help them adhere and flow smoothly, if you have the option of using a brushing glaze that would go on easier than a dipping glaze. Are these the same pieces that you were asking about stilting when firing?

If you want to apply the underglazes to bisqued clay then you could either glaze them after the underglaze has thoroughly dried on your 05 bisque or rebisque them to a low temperature to burn off the gums etc that they contain. Don't need to go very hot, just to around 820C (approx cone 015). 

Thank you for this very detailed explanation, Min. I honestly appreciate it so much. I feel a bit stupid, firing the underglaze to vitrification temperature - I know that glazes don't take well on vitrified clay so I'm not sure why that completely went over my head. With regards to these pendants, they actually look really nice as they are without an overglaze, so for this batch I will leave them as is:

20190923_074045 by Rachel Brown, on Flickr

If I wanted to repeat this process to accomodate a transparent overglaze in the future, do you think the following would work:

1. Bisque fire casts to Cone 05

2. Paint on underglaze and fire to cone 015 to burn off impurities

3. Paint on overglaze and fire to cone 7 (will mature at 6) to create final, vitrified piece.

I don't think I could do liambesaw's suggestion of painting onto greenware, as the pieces are so fragile before bisque firing - it is very easy for example to chip off an ear on a pendant once dried.

With regards to crazing, the supplier has recommended this exact overglaze to pair with the slip I am using from the same supplier. It is designed specifically for porcelain. So hopefully it will be okay, so long as I fire to the correct temperatures.

[Roberta12]

@SweetheartSister I have used underglazes in a variety of ways.  I agree with what you said about not applying the underglaze in the greenware state if your piece is small and fragile.  I learned that the hard way.  Perhaps I have been doing it wrong but I do not do your step #2 .  I put the clear glaze on top of the underglaze ( make sure your underglaze is dry) and fire to cone 6.  Done.  

Recently I have been doing some newsprint slip transfer work and I have been applying that to greenware pieces.  Larger pieces like cups/planter/bowls, things that are not as fragile as delicate animals, jewelry or buttons (I use a lot of porcelain too!) And I have also started using underglaze on some larger pieces in the greenware state and doing some carving.  Then bisque, then apply clear glaze and fire to cone 6.  

I like underglaze for it's ability to be used in a number of ways.  I have applied underglaze for years to bisqueware.  Nary a problem.  But....I am trying to do more of the underglaze work on leatherhard/greenware simply because it speeds up the final glazing process.  

Your animals are beautiful! Wonderful detail!  

Roberta

[Min]

6 hours ago, SweetheartSister said:

I feel a bit stupid, firing the underglaze to vitrification temperature - I know that glazes don't take well on vitrified clay so I'm not sure why that completely went over my head.

Oh goodness don't feel stupid! Perhaps it's because you also work with earthenware and sometimes the bisque is hotter than the glaze firing with that. It's a never ending learning curve with ceramics, we've all done the occasional blunder.

6 hours ago, SweetheartSister said:

If I wanted to repeat this process to accomodate a transparent overglaze in the future, do you think the following would work:

1. Bisque fire casts to Cone 05

2. Paint on underglaze and fire to cone 015 to burn off impurities

3. Paint on overglaze and fire to cone 7 (will mature at 6) to create final, vitrified piece.

Yes.

I agree with Roberta,  try brushing some of the glaze over dry but unfired underglaze and see if it smears. (test piece) Would save a step if you can glaze over dry underglaze.

Even though your supplier said your glaze will work on the porcelain I would still check with some scrap clay how the glaze fits before committing a load of your adorable creatures to it. Try different thicknesses of glaze to see how it looks over the underglazes on test pieces too. Sometimes the underglaze colour shifts when glazed, depends on the chemistry of both the underglaze and the glaze.

[neilestrick]

Try applying the underglaze to bisque and then applying the clear glaze without firing on the underglaze first. It may work fine. There are enough binders in the underglaze that you should be able to brush on a clear glaze without messing it up. If it does mess up the underglaze, or you're dipping the clear glaze, then you'll want to burn out the binders before glazing. It would be nice if you didn't have to fire 3 times, though, so try applying the underglaze when your pieces are leather hard or bone dry. If you're used to working on bisque, then bone dry is probably the way to go because it will be more porous like bisque.

[SweetheartSister]

On 9/23/2019 at 11:14 PM, Min said:

Oh goodness don't feel stupid! Perhaps it's because you also work with earthenware and sometimes the bisque is hotter than the glaze firing with that. It's a never ending learning curve with ceramics, we've all done the occasional blunder.

Yes.

I agree with Roberta,  try brushing some of the glaze over dry but unfired underglaze and see if it smears. (test piece) Would save a step if you can glaze over dry underglaze.

Even though your supplier said your glaze will work on the porcelain I would still check with some scrap clay how the glaze fits before committing a load of your adorable creatures to it. Try different thicknesses of glaze to see how it looks over the underglazes on test pieces too. Sometimes the underglaze colour shifts when glazed, depends on the chemistry of both the underglaze and the glaze.

Thank you Roberta12, Min and Neilestrick; it seems that the verdict is to try the overglaze on top of dry underglaze so that I don't have to fire 3 times. I'm going to do some test pieces and see how that goes.

In the future I would definitely like to put a clear overglaze on some of these designs, but I am still fearful of crazing happening even a year down the line, which is what has happened on some of my earthenware pieces. Is there a fool-proof way to be sure that the glaze fit is definitely okay?

I actually contacted a factory recently about batch producing some of these pendant designs in earthenware, but when I asked if they would eventually craze, they said definitely. I also spoke to my kiln supplier on the phone and he said that porcelain is a much safer option regarding crazing because it is a much less porous clay than earthenware.

As I'm selling these, I just want to be sure that they will last a long time so that my customers are happy

[liambesaw]

You can do the freezer to boiling water test for crazing to see if the glaze is a good fit.  Just freeze some samples and then drop them in boiling water. This will cause them to craze instantly if they're ever going to craze

[SweetheartSister]

1 minute ago, liambesaw said:

You can do the freezer to boiling water test for crazing to see if the glaze is a good fit.  Just freeze some samples and then drop them in boiling water. This will cause them to craze instantly if they're ever going to craze

Oooh okay, that sound straight-forward enough to do. Is it important to make sure that you're testing with the exact item, e.g. using a cast out pendant to test rather than a different casting/piece of clay which is a different size? Like if I were to glaze a mug and a pendant in exactly the same way and fire them for the exact same schedule, would the results be different due to scale of the items?

It makes the most sense for me to cast out some pendants as tests, but I'm just really curious as to what the variables are here

[liambesaw]

It wouldn't be different due to scale, but could be different due to the shape.  A mug has an inside and outside, a pendant is glazed on one side only.  This puts different pressures on a glaze.  I do my freezer to boiling test with mugs because all of my forms have an inside and outside that are glazed.

[SweetheartSister]

18 hours ago, liambesaw said:

It wouldn't be different due to scale, but could be different due to the shape.  A mug has an inside and outside, a pendant is glazed on one side only.  This puts different pressures on a glaze.  I do my freezer to boiling test with mugs because all of my forms have an inside and outside that are glazed.

Oh, that makes sense! So put in the freezer for a day, take out and straight away pour over boiling water in the sink?

Do you carry out this test with every single batch of mugs you fire, or just the once if you're duplicating the variables such as firing time/temp, layers of glaze used, same glaze and same clay etc?

[oldlady]

sister,   you might want to just bisque the pieces and use Stroke and Coat colored glazes.  fire to cone 6 if that is the temp you want, and they are finished.  you will have to cover the entire piece to get a glossy finish which means using white S&C as well as the colors you choose.