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PaulR

Number Of Firings

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Hi,

I have made some pots for plants and used to do a simple 04 bisque and a 6 firing.  Now I am using underglazes on the greenware, firing to 04, using a clear glaze and retiring at come 6.

 

I tried using some crater type glaze but it needs to be fired at 05.  I hate to do 04, then 6, then 05.

 

What happens if I do an 04 then use the crater glaze and fire at 05 as a final firing?  I am using cone 5/6 clay.  I know that means it will not be vitrified but what effect

will that have on the pots?  They will not be outside so no freezing temps involved.  Will water seep through when the plant is watered?

 

If that is not a good option can I let them dry longer and eliminate the 04 and use the underglazes and the clear glaze on the dry greenware and fire to 6 then do an additional 05?

Can you use the clear glaze on greenware?

 

attached is an example

 

Thanks for any advice.

post-59342-0-53723400-1487795777_thumb.jpg

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Your first glaze idea of 05, 6, 05 should work.

I have not specifically done this with crater glazes but I have with texture glazes and lichen glazes.

The last firing at 05 is not hot enough to hurt a glaze that is fired to Cone 6.

 

That said ... I WOULD NOT DO THIS WITH FUNCTIONAL PIECES. ( sorry, but yes I am shouting this. : - ) )

 

Underfiring the whole piece would be awful for a planter ... you will soon have green mold all over the surface.

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Another soloution would be to find a crater glaze that works at cone 6. Or switch to an earthenware that matures at cone 04, and find a clear that works at that temp, too. I find the binders in commercial underglazes resist glaze application unless fired to bisque temps first, and if it were me, I'd be reluctant to eliminate the bisque. And your pot will weep if it's only fired to bisque temperatures and it says it matures at cone 6.

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I would aim to have the bisque and crater glaze fired at the same temperature, say bisque to cone 05.  Then you could fire the crater glazed items along with the bisque items. 
 
I also would try firing the crater glaze on a test object to cone 6 and see what happens. 
 
LT

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Potters once fire cone 6 clay with glaze applied to greenware  (might even have one or two on the forum who could give some pointers).  So, it is possible to eliminate your first bisque.  Will take some trial and error to get applications and firing right. 

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I already do the 04, 6, 05 firing so I know it works.  Just trying to save some time & money doing 3 firings each time.  Not to mention the increased ware on the kiln.

Well, if you try the elimination process ... bearing in mind I could be totally wrong here ...

 

You cannot underfire the clay or you will get seepage and mold ... so option is to change clays.

Eliminating the bisque and once firing to 6 with glaze inside and outside is very tricky and likely means reformulating your glaze which could mess up your results for a while.

Firing your crater glaze in the same load as your next bisque should work if you keep the shelves separate.

 

Your costs of separate firings might be worth not loosing time wandering down those paths. As to wear and tear on the kiln, I don't think Cone 05 firings are very hard on them.

Sometimes multiple firings are the only way to achieve the exact look you are going for.

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If you want vitrification, make a cone 5/6 crater glaze. Adding silicon carbide will cause cratering in many glazes. Use a really fine mesh, like 800 grit, which you can get from lapidary supply stores online.  A good starting point is 0.5% by dry weight.

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Paul i look at your picture and i can't quite make out what you mean when you say craters. 

 

when i think of crater glaze i think of volcanic glazes with a lot of cratering. 

 

did you create the cracks or did the glaze create the cracks.  or are the flecks the craters you are talking about. 

 

can you post a picture before the crater at ^6 please. 

 

just curious. i know lucie rie got pitted surfaces adding rice to her slip (i am in the process of trying that myself - with rice and cornmeal). have you ever tried that to see if you like the look. that would eliminate a 3rd firing. 

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Oldlady, did you mean to say sodium silicate instead of silicone (silica) carbide? I've seen silica carbide use in and on a glaze, but I'm not sure how you would then "stretch (sic) the pot from the inside." I've also seen sodium silicate used on raw clay, dried a bit and then stretched, but never on a glaze. Guess that could be done in once fire glazing, though the pot would have to be in the leather hard state to be stretched after glazing. Now that might be something to ponder…hmmmm

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I would think that using a ^5/6 clay to ^05 would open yourself up to "weeping", not actually a leak, but a dampness on the outside of the pot all of the time that would gather wetter on the floor/table. As Neil has said, maybe better to use a volcanic ^6 glaze and develop your craft from there. Here is a link to a starting point. .. . 

 

https://glazy.org/recipes/4454

 

 

best of luck, 

 

Pres

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thanks, sweet fred.   it was late and the brain was trying to tell me to go to bed.  

 

i have not seen that technique used except over slip or just the bare clay but i do single fire.  the thought of trying it over glaze does not appeal to me at all, i want my work to be attractive and the mix of raw clay in the middle of a glazed piece does not sound good to me.

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The example I showed does not have any crater glaze on it.  That example is one I used sodium silicate to create the cracks.  This pot was fired to 04 then clear glazed and refired to cone 6.  I would like to add a rim of crater glaze at the top but that glaze is for cone 05 so my original question was trying to not have to do 3 firings.

 

I have tried added the carbide to various cone 6 glazes but they never came out right.  All the formulas are for making the glaze from scratch whereas I would prefer to add the carbide to a store bought glaze.  So when Neil says add .05% I am not sure how much that would be for an existing glaze?

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 So when Neil says add .05% I am not sure how much that would be for an existing glaze?

 

Take a small bisque shard,

add a spoon full of commercial glaze to that shard. 

When the glaze is dry, weigh the shard + dry glaze. 

 

The difference between these two weights will be the a reasonable effective dry weight per spoon full of the commercial glaze.

 

Use that as the amount of dry solids per spoon full of wet commercial glaze. 

 

  LT

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OldLady,  I understand that many potters would not be interested in creating "cracks" in their work but using that method does not mean the work is not attractive.  It depends on the clay and glaze used.  Here are some of my work and I would not classify them as unattractive.  These are pots used for various cactus.

 

Fred Sweet,  Maybe I am misunderstanding your comments so sorry if I am.  Once I make a pot I apply various underglazes while the pot is still moist.  At least one of those underglazes I have added  sodium silicate to the jar.  As soon as the under glazes dry I can stretch the pot easily.  Once in a while I might add the underglazes and then just brush the silicate over it but usually I have added it to one of the underglazes.

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A less precise way to do it would be by guesstimating it. Many glazes are between 70 and 80 grams of water to 100 grams of base glaze. 

 

To keep the math as simple as possible you could weigh out 180 grams of liquid glaze, of that a guesstimate would be that it contains 100 grams glaze, 80 grams water. So to add Neil’s amount of silicon carbide you would add 0.5 of a gram of silicon carbide to that. I would actually do a series of incremental increases to test a bunch of amounts. Start with adding 0.5 grams, dip a test tile, then add another 0.5 grams dip another test tile and keep going up by 0.5 until you get to about 4%. Could decrease the amount of base glaze to half that amount to save some waste glaze (and cut silicon carbide additions by 1/2 also). 

 

 

The weight of the water is not going to be spot on but it’s the ratio of wet glaze weight to silicon carbide that is going to be able to be replicated when you find a % that works. Just do a small test tile dip each time as the volume of base is going to be decreased with each subsequent dip and throwing the ratio off slightly.

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Yes.  if the solids in a spoon full of wet glaze dries to one oz  and you need three spoons worth of glaze you would use a 0.005 x 3 oz of silicon carbide.  (or about 0.43 grams )

 

Min's procedure is also reasonable approach.   I seldom use commercial glazes, so I have no insights on the solids to liquid ratio for them. 

 

LT

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