Porcelain Once Fire Help

[ThatTeenPotter]

Hi everyone, I've always wanted to do pottery and the time has finally come. I got a second hand kiln and a ton of supplies a few days ago but now I have a couple of questions.

Until I get my sculpting clay and mid fire glazes, I'm using cone 6 porcelain slip and making molds with it. It didn't come with any mid fire glazes, only low fire cone 06 glazes. My question is, could I apply the lowfire glaze to the porcelain greenware and do a once fire at cone 06?

If someone could let me know I'd appreciate it. Thank you

[Bill Kielb]

If your porcelain is cone six, firing it to cone 06 will only sinter it which means not really melted  or mature as they say so even if this glaze clay combo works, it will not be very sturdy And leak water.. Once fire will also require a bisque schedule, meaning slow and likely about twelve hours to burn out the organics and chemically combined water as well and your glaze will need to go through this and hopefully make it without defects.  You could try firing the clay to cone six meaning a traditional bisque and glaze fire of the clay or even a once fire of the clay to maturity then glaze the object with the 06 glaze and glaze fire to 06 to mature the glaze.

Industry does this to weed out any potential failures from the get go before they waste the energy and product of their glazes. Problem is, applying glazes to a fully fired object is difficult to get on evenly and to proper thickness. All pretty complicated hybrid stuff if you are new. I would advise get your clay and glaze matching first (cone 6 clay, cone 6 glaze) and do traditional bisque then glaze firings.  The bisque fire removes organics and combined water and allows the glaze to be absorbed nicely while  making the ware slightly sturdy enough to handle for decoration.  Once fire would be a step up in experimentation and complexity with respect to the Glaze / clay combination and the piece turning out.. Leave the down firing approach  of a lowfire glaze over a mature midfire product to industry, they do this well.

over time you will understand the firing processes better with respect to glaze maturity, clay maturity and can then experiment from there. None of this hybrid stuff is very suitable for a new potter  in my opinion.

[ThatTeenPotter]

Thank you for your response.  My porcelain slip says to fire to cone 6, I assumed that meant cone 6 glaze and a traditional 04-08 bisque.  I was under the impression that was what you bisqued every clay body to, is porcelain the exception?  Do I really bisque and glaze fire my porcelain to cone 6?  I'll think I know a good bit when in reality in a new Potter and any new piece of information scrambles my brain lol. 

Okay, now as I re-read what you said about applying glazes to a "fully fired" object at cone 6 that leads me to believe again that everything is bisqued to 04-08 and then, for example, my porcelain would be glaze fired to cone 6.  It wouldn't make sense to fire to the same temperature twice only to put glaze on the already fully fired clay and put it in for a second time.  Its late and I need some sleep, my bad lol.

I'm gonna wait until I get my cone 6 glazes and clay and start experimenting from there.

Sorry for the confusion , I'm a new Potter with a lot of willingness to learn.  Thanks so much!

[oldlady]

teen,  once you sleep and your thinking clarifies, you might see that you said "traditional 04-08 bisque".    just to make sure you understand it, cone numbers starting with zero run from large to small and increase in heat up to cone 1.     so, cone 08 is cooler than cone 04.    look at a cone chart to see the numbers.

[neilestrick]

Yes, always bisque at low fire temps. 04 will give you better burnout than 08 and avoid potential problems with the glaze firing.

Some clay and glazes do okay with once firing, others do not. But in general, low fire glazes do not do well on porcelain. You'll probably get crazing or other issues, and the clay will be weak. If you want to do low fire work, then get clay designed for it.

[ThatTeenPotter]

Thank you so much everyone, as I said, I finally got my kiln to be able to start my journey into pottery.  I wasn't expecting it to come with things such as porcelain slip and low fire glazes so that made me a little too eager to start creating projects.  I'm piling my order together of a mixture of low fire clay (since I already have low fire glazes), mid fire stoneware, and some cone 6 glazes.  Once I get that, I can start creating pieces and finding my own art style!  I do generally understand the cone chart, I plan on starting with cone 06 as a good medium bisque temperature and depending on the results I can experiment with 05, 04 because I've heard some people don't get as much pinholeing at 04.  Thanks again everyone, I'm happy to be apart of a community of experienced potters that can help me answer my silly newbie questions!

[Bill Kielb]

4 minutes ago, ThatTeenPotter said:

I'm piling my order together of a mixture of low fire clay (since I already have low fire glazes), mid fire stoneware, and some cone 6 glazes

I will only mention this once because most of us have made these errors over time.

1. OopsI fired my lowfire clay to cone six and ........... what a mess!
2. Hey I thought that was 06 I was firing to in the bisque,  cone 6! Oops, this stuff is really hard to glaze!

[ThatTeenPotter]

I plan on having a notebook system to hopefully avoid all of that mess!  I've got my order almost wrapped up, some lowfire clay to go with my lowfire glazes I have.  Some Bella's blend versatile clay that can be glaze fired at 06 or 6, and some stoneware clay.  I'm getting commercial Mayco Stoneware glazes for now but now I'm left with a few questions.

I know the Mayco glazes will be compatible with the stoneware clay, but is it going to be compatible with the Bella's blend because it doesn't specify it's Stoneware?

Another is, I already have some porcelain project's but can I use the Mayco Stoneware glaze on the porcelain?  I understand it contradicts each other but I have yet to find glaze specifically for porcelain.  The glaze says it can be fired from cone 5-10 so I'm thinking I can use it on my porcelain slip, which says to fire to cone 6.

One way I could find out it to test it but I figured I'd ask some seasoned potters first!

Thanks again to everyone who's helping me start my journey in pottery!

[Bill Kielb]

I’ll give you my take,  in general decent claybodies usually have a small range they fire in as in cone 5-6. When you fire a clay to maturity it becomes less porous and as strong as it can be we call this maturity or fully vitrified. Clays with large ranges generally are too porous at the low end of their range so anything with more than a cone or two max difference in my opinion is often too porous at the low end of their range even for mugs. They leak, will not be very strong, and rely on a perfect glaze without imperfection to function.

so a blend that fires 06 to 6 probably Is ok for functional ware only when fired to 6  (maturity) or it’s a typo and somehow they are referencing the bisque temperature and it’s actually a midfire clay or cone 6.

Lowfire clays 04/05/06 generally will always be too porous for functional work and rely on a perfect glaze surface without defect to be water proof. Match lowfire clay with lowfire glazes they are most suitable for non functional ware.

Midfire clays  and glazes, generally cone 5/6 are popular and generally can be used for functional ware, match midfire Clay with midfire glazes and fire to maturity when possible.

Matching a glaze firing range with a clay firing range will not mean it matches in coefficient of expansion so other defects can appear such as very fine cracks called crazing. The only way to know if the combination is expansion  compatible is to test Fire together. There Is no other way to know in advance other than someone telling you it works for them with the same combination, and even then it’s not for sure so test first.
porcelain is another form of clay, but still clay. It is generally white and smooth with no grog. Some find it more difficult to learn how to throw as it may perform in a less plastic manner. In general porcelain is stronger than stoneware assuming fired to maturity even though stoneware sounds strong. It’s just a name.

In the UK folks often refer to stoneware and mean what we know as earthenware type clay. Generally lowfire, porous, leaky and not the strongest composition. Think terra-cotta.

The earth is cone 10! All our clays and glazes are Basically the product of the earths geology which naturally melts about cone 10. Maybe on another planet the geology melts higher or  lower than ours. So basically all glazes start as cone 10  (On planet earth) and certain fluxes have been added over the years to make them melt at cone 6 as well as lowfire,- cone 04. You may hear boron one day, and this is a popular way to make our cone ten geology melt earlier.

cones - cones are made of glaze and reflect the amount of heatwork done on a clay and glaze. Cones are affected by time and temperature just like baking a cake at 350 degrees for thirty minutes, time at temperature is how we bake a cake to tasty completion. Same for clay, cones will indicate that a certain amount of heatwork has been done so you know your clay and glaze was fired properly.

I need to stop now, all sounds tough but as you do it, you will learn many things. In the beginning it is difficult but in the end simple and very rewarding. Ceramic arts allows for great creativity in construction, decoration, glazing, firing and even some simple chemistry. Most find it extremely rewarding and a challenge to continue learning.

Ok, hope that helps a little, youmoughtnto see lots of opinions pop up here.

 

 

 

[ThatTeenPotter]

All of this is definitely a challenge but I've never been the type to shy away from that, especially when this has always been a dream of mine.  I'll make sure to match clay bodies with it's respective glaze and if something fails atleast I'll learn from my mistakes and turn that into a learning experience.  I didn't quite realize how expensive shipping would be on an online order so it looks like I'll be making a drive to the nearest clay shop around me, which is a decent drive.  Thanks again to everyone who helped me out, especially Bill.  I believe I'm done with my pesky questions for now but if any more come up I know where to post em!  

[Chilly]

47 minutes ago, Bill Kielb said:

In the UK folks often refer to stoneware and mean what we know as earthenware type clay. Generally lowfire, porous, leaky and not the strongest composition. Think terra-cotta.

In the UK stoneware is ^6. 

Terracotta usually refers more to the colour, and to standard clay plant pots.

[Min]

I've noticed a few claybodies from England that are called Earthenware  with broad firing ranges. A quick Google search brings up a couple examples of these bodies, Potterycrafts Professional Super White Earthenware is rated up to 1220C / 2228F. Scarva White Earthenware, rated up to 1240C / 2264F. Clayman Studio White Earthenware rated up to 1220C / cone 6. I think the problem comes from having broad firing range claybodies, sure at the lower temperatures those would be called earthenware but terms get muddied when we think of earthenware going to cone 6.

[ThatTeenPotter]

I will get their opinions on everything before I make any final choices!  

On a different note, I got this kiln second hand and it's in great condition other than normal slight cracking on the bottom and a few glaze spots.  I've read that you scrape off/dig the glaze out or else it can continue to eat through your brick.  I tried attaching a picture but it won't let me, it's about 3 spots, one is on the surface and the other two are slightly into the kiln brick.  Please let me know what you think I should do.  I've heard that some people fill it in with kiln mortar or kiln wash.  I'm not completely sure which direction to take here.  Thanks

[Min]

Yes, you need to remove all glaze from the bricks. Glaze on soft kiln brick acts like a cavity on a tooth, can't just cover it up, needs to be completely cleaned out. Remove all the glaze with small picks or chisels, whatever you have that won't mess up the bricks more than you have to. If it's a kiln you can take apart into sections and the spots are in hard to reach areas then take it apart to do the work. Where are the glaze areas in the kiln? Floor should have kiln wash on it but not on the walls of the kiln.

To attach a picture try emailing it to yourself, choose a smaller image size, then you should be able to post that smaller image. 

[Bill Kielb]

14 minutes ago, ThatTeenPotter said:

I'm not completely sure which direction to take here.  Thanks

Carefully removing it / digging it out with the least damage to the brick is common else it continues to eat into the brick. Patching is fine with one exception if the patching material will touch an element then maybe nothing or just kiln wash. Most patching materials specifically discourage contact with an element.

[ThatTeenPotter]

I was able to crop the photo to be able to upload!  It's right underneath the kiln sitter.  The floor hasn't been kiln washed but the shelves I got with it have.  There seems to be a thick layer on the shelves that's peeling off.  Do I scrape that off and apply new wash?  

[Min]

After getting all the glaze off the floor apply a couple thin coats of kiln wash to the floor only, not the sides of the kiln. Don't get the wash on the elements. For your flaky kiln wash on the shelves you'll need to wear a P100 mask/respirator and take the shelves outside and scrape and grind all the flaky wash off. Grind out any glaze drips on the shelves.

A good kiln wash recipe is 25 alumina hydrate, 25 calcined epk and 25 epk. Mix it up to low fat milk consistency, again wearing the P100 mask with dry materials. (don't need the mask on once the wash is mixed up) You can use a wide brush or a paint roller to apply, depends on how big your kiln and shelves are. (I use a mini roller) The first coat is going to suck right into the bricks, wait for that to dry then apply a second coat. When you apply the wash to the shelves don't get it on the sides or bottom of the shelves. Again, 2 thinner coats are better than 1 heavy coat. Wipe the edges of the shelves with a damp sponge when you are done to get any drips off. Let the shelves dry then they are fine to use with your first bisque firing.

[Chilly]

I don't use the floor to put stuff on.  I use the smallest shelf posts and a shelf.  Easier to apply batt wash on a shelf than on the floor.

[ThatTeenPotter]

I'm going to get the glaze drips off the bottom of my kiln floor, apply a couple coats of kiln wash.  Scrape and re apply kiln wash to my shelves.  I'm assuming the woman who owned the kiln before me fired directly on the floor but I plan on using the smallest posts and a shelf.  I've heard it's a big no no to fire directly on the floor.  I've got a bunch of stilts that I'll use for low firing since that will allow me to glaze the bottoms and I'll just wax the bottoms for when I'm firing stoneware.  Thanks everyone!