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PaulR

Sealing Bisque Pots

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  I have been making a lot of pots for my plant collection and normally I glaze all the pieces.  I recently made some pots combining 3 different color clays, a dark brown, gold (really light brown), and a buff.  Once the pots dried and were bisque fired, the combination of the 3 colors looks great so I do not want to ruin the look with glaze.

 

  I picked up a clear glaze and fired 2 of the pots as a test.  I did one coat on the first pot and two coats on the second pot.  These were fired at cone 6.  On both pots the clear glaze washed out the nice colors of the bisque and the entire pot just has a boring gold kind of look which I do not like at all. 

 

  I was reading about sealers and was wondering if I used a matte clear sealer if the clay colors would remain the same?  Also, would I just do an 04 bisque fire and that's it or should I do them in a cone 6 firing?  Any advice is appreciated.  Thanks!

 

Paul

 

 

 

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There's nothing wrong with leaving them as bisque, but they will be porous and the plants will require watering more often. A fair amount of the color loss can be attributed to the clear glaze. I would try firing them to cone 6 without glaze.

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Another option would be to apply glaze on the inside of the pots to help them hold water, but apply a soda ash wash to the outside. The soda ash wash will give the clay a bit of a sheen, but allow it to keep its natural color. Try mixing 1 cup hot water (hot enough to dissolve the soda ash) and 1/8 to 1/4 cup soda ash. I usually take a piece of sponge, dip it in the mixture, wring it out, then wipe down the sides of the pot. Wear a latex glove on the sponge hand when applying the wash.

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With many clays firing hotter changes colors with or without glaze.

 

You like the color fired at bisque then coat the inside with a low fire clear and refire it at Cone 05.

You keep the color, get a bit more of a seal.

Cone 05 is cooler so your color should be fine.

I would not put the pot on your favorite wooden table without a plastic protector or trivet ... but you can always do tests to see how much it leaks and use it with appropriate care.

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Thank you for the replies and ideas.  So would applying a sealer to the outside not accomplish the same thing as applying the clear glaze to the inside?  I hope that is not a stupid question.

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The sealer on the outside would accomplish the same thing, but the interior glaze would be a little more permanent and/ or durable.  Also, if you just sealed the inside with glaze, you wouldn't have to worry about a sealant on the outside, altering the colors that are already there.

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Why not try and fire a test piece of the blended clays to ^6 without glaze and see how the colours change?

 

Are the pots for indoors or out? I have a huge outdoor planter I made with ^10 sculpture clay and single fired to ^6 with no glaze, it's not vitrified but it doesn't matter since it's outside.(I just protect it from freezing during the winter) If the clay is really underfired then it's likely that over time minerals from the water and any fertilizers you add  will seep through and discolour the outside. It might make a nice aged patina effect. Glazing the inside only of an  immature body might work but unless it is a really tight fitting glaze with no crazing the moisture will seep through anyways. 

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The pots will be in my greenhouse.  My main concern is because I have so many plants in there when I water I just use a shower hose handle so every part of the pots get wet.  I'm sorry, I do not know what you mean by vitrified.

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Vitrification is when the clay particles fuse together, making the ceramic water tight.

 

Low fire bodies, are generally not fired up to vitrification, so they are still porous. They are fired to the point, that the chemical water is driven off, and the particles "lock" together, making the ceramic strong.

But, there is still space around the particles for water to seep through. Glaze can be used to seal the ceramic, but only if the glaze and ceramic body match up well enough. If they don't match up just right, the glaze will have small cracks, which will still allow water to seep through.

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I wouldn't worry about a glaze or firing to ^6. As long as they are not too fragile for your use then you don't need to go as hot as ^6. Perhaps around ^2 or ^3 would be a compromise between strength and colour aesthetics.

 

Think of the terracotta pots that are used in gardens everywhere. They are not vitrified but no problem soaking them with a hose or outdoors in the rain. I would just avoid freezing temperatures since then they could crack from moisture expanding in them. 

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