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Burnishing Methods


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#1 gkillmaster

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Posted 16 April 2014 - 07:04 PM

I had the idea to stop using glazes and instead try to burnish my pieces. I'm making cups to hold liquids so I have to find some other way and found out that burnishing can create a non-porous surface.

Has anyone tried using burnishing to seal a food vessel? What would be ideal is for me to make my cups, burnish them on the inside to seal them, then fire them at somewhere between cone 6 and cone 10 or whatever temperature the clay I choose beings to become vitreous. I'm hoping to use a warm colored stoneware for this. That way I can only do one firing and be done... :)

 

Any experience or tips is greatly appreciated!

 

Greg K.



#2 Colby Charpentier

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Posted 16 April 2014 - 07:32 PM

I have not tried burnishing to seal a body to use as a serving vessel, however I know that when pit-firing, we try to use a low bisque, as higher temperatures will tend to "remove" the burnished surface...  I haven't seen it, but I've always been told to not burnish anything being fired above bisque temperatures....



#3 bciskepottery

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Posted 16 April 2014 - 07:49 PM

Glazing the inside of a food vessel provides protection against staining (tea, coffee, tomato sauce and other acidic foods, etc) and potential bacterial growth. And, I can almost cringe at the sound of a metal knife moving across the surface of a burnished, but unglazed plate.

I've heard of applying and burnishing terra sig to the foot of low-fire/earthenware bowl or mug to seal the clay body and reduce porosity, but that surface alone is not conducive to safe food preparation/serving.

Above low fire temperatures, you lose the burnish from the surface.

#4 Stellaria

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Posted 16 April 2014 - 08:59 PM

Is this for everyday use, or for historic display / experimental archaeology purposes?

#5 Marcia Selsor

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Posted 17 April 2014 - 06:08 AM

If the clay is vitrified it should not leak. As mentioned about, the surface may not feel appropriate for food or drink and may actually be a health hazard. The Romans used terra sigilatta to seal some fine pottery cups and bowls. The terra sig loses its sheen if fired hotter than cone 09. It may still seal. I can't say about that. As always, you'll need to test whatever type of clay you plan to use.

 

Marcia



#6 neilestrick

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Posted 17 April 2014 - 08:48 AM

If you're firing to vitrification, then there's no benefit to burnishing in terms of making them water tight. The vitrification does that already. Burnishing may give you a smoother surface that is more hygienic and easier to clean, however as others have stated, you'll burn off the shine if you go above low bisque temps, and it still won't work as well as glaze. You'll also have a very hard time burnishing the inside of a vessel. If you plan to sell your work, it will be very difficult to convince people that an unglazed interior is water tight and food safe, even if it is.

 

Is there a reason you don't want to use glaze? If you prefer the look of an unglazed surface, you could always leave the outside of the pot unglazed and use a subtle matte glaze on the inside.


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#7 Joy pots

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Posted 17 April 2014 - 11:39 AM

I burnished the top half of a round pot, bisqued at cone 06 then glazed the bottom half & fired to cone 6. The burnished top half is still smooth & the clay is stoneware.

So in actual fact burnishing at cone 6 seems to work.

#8 gkillmaster

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Posted 17 April 2014 - 11:44 AM

thanks everyone. Totally helpful information and questions brought up here! I love this place!

 

I'm just really wanting to pursue raw clay forms after being on a glaze binge. I'm going to do a series of experiments with this. A lot of my work will be intended to be used for drinking liquids and I'm just exploring this out of curiosity. I'm curious wether I can make a raw clay vessel that is not porous. I realize the coarseness of the surface may not be appealing, but then again, I might get some interesting results. If it doesn't work out, then they could be used for plants or other non-food containers.

I may go back to glazing at least the insides...

 

Good to know that burnishing is mostly a low-fire technique!

 

The other thing I thought I might try is to use a glaze slip and apply it to the post before it dries. And not doing bisque but firing one time to cone 6 or so. Not sure if that would crack the pot if it's melting temperature is very different from the clay body?

 

I'm trying to find a way to get good results in a single firing. That is sort of the goal. And hopefully I will stumble across a technique that can offer aesthetically pleasing results as well :)



#9 gkillmaster

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Posted 17 April 2014 - 11:46 AM

I burnished the top half of a round pot, bisqued at cone 06 then glazed the bottom half & fired to cone 6. The burnished top half is still smooth & the clay is stoneware.

So in actual fact burnishing at cone 6 seems to work.

 

Wow! this is great to know! I will try this with a single firing to cone 6 after burnishing to see if the burnishing holds. thanks alot Joy.



#10 neilestrick

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Posted 17 April 2014 - 12:08 PM

 

I burnished the top half of a round pot, bisqued at cone 06 then glazed the bottom half & fired to cone 6. The burnished top half is still smooth & the clay is stoneware.

So in actual fact burnishing at cone 6 seems to work.

 

Wow! this is great to know! I will try this with a single firing to cone 6 after burnishing to see if the burnishing holds. thanks alot Joy.

 

 

The pot can remain very smooth at cone 6, but the polish will burn out. You lose the sheen.


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#11 Stellaria

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Posted 17 April 2014 - 12:11 PM

I know Simon Leach has some video clips on "raw glazing" where he glazes greenware for a single firing. Might be worth looking up.

#12 gkillmaster

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Posted 17 April 2014 - 12:15 PM

I know Simon Leach has some video clips on "raw glazing" where he glazes greenware for a single firing. Might be worth looking up.

 

great! thanks. I will...



#13 gkillmaster

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Posted 17 April 2014 - 12:16 PM

 

The pot can remain very smooth at cone 6, but the polish will burn out. You lose the sheen.

 

 

interesting... thanks Neil.



#14 Colby Charpentier

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Posted 17 April 2014 - 03:53 PM

If I need a high fire bare clay surface to be smooth, I'll sand it down by hand, especially with wood or salt fired surfaces that can sometimes need some love. It skips the burnishing step, but still gives a really nice surface. If you start to get into polishing, there are some possibilities also....    There's an Alfred undergrad doing polished stoneware vessels, and he's achieving some really nice surfaces. I'm not sure who else does this, but just an idea...



#15 gkillmaster

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Posted 18 April 2014 - 12:11 PM

am mulling all these suggestions over. thanks everyone! I may do a test using all these methods just to get a feel for what works.! thanks again!



#16 flowerdry

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Posted 18 April 2014 - 08:07 PM

Don't forget to share Gkillmaster, I would love to hear about what you discover.  Also, it would be nice to know a little more about you.  Please consider fleshing out your profile.

 

Love the hat.


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#17 gkillmaster

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Posted 19 April 2014 - 12:34 PM

Don't forget to share Gkillmaster, I would love to hear about what you discover.  Also, it would be nice to know a little more about you.  Please consider fleshing out your profile.

 

Love the hat.

 

O sure. I would love to share what I end up with. thank you!



#18 alabama

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Posted 21 April 2014 - 11:54 AM

If I had to guess, I think burnishing pottery to make it water tight is a myth started by a modern potter examining the

exterior of pottery from the Mississippian culture.  On the basis of "Form follows Function" the early potters are probably copying

the surface of an object they're familiar with - the common gourd.  We know they had and used gourds since they made clay copies of

them that are "gourd effigy pots".  The paste they used was made up of clay, organic materials, sand/dirt, and crushed shell so you

can see that the pottery was quite porous.  Unglazed pottery has one use, to hold more liquids than leak out.

 

All the pit-fired unglazed pottery I've eaten from has the common trait of tasting like burnt clay...regardless of how many times

they're used.  Any bowl used for food gets moldy, whether they are rinsed out or not.  The mold issue can be remedied by refiring the pottery.

All the pots where drinks were made became micaeous, from the natural inclusions of mica in the clay.  It seemed ok to drink since the liquids

were brought up the boiling temperatures on the pit fire.

 

In the Southeast United States, when the Mississian culture collapsed pottery became smooth but not burnished until about 1775 when

burnishing returned, probably to copy the glaze finish from European trade items. 

 

    Different cultures make the pottery for different reasons.  Some South American Amazonian cultures grind tree resins into a powder and when the pots come

off the fire, the mixture is dusted onto the exterior making them appear glazed.  In Peru, the clay has so much lead ore in it that when

the pottery is fired it has a natural lead glaze...I think it is called plumbate pottery.  Some people who use this pottery live to as old as 34.!

 

But since you plan to use pure clay and an electric kiln, its wide open for you to experiment with burnishing and such.

Make lots of things with notes on what steps were taken.

Good luck to your endeavers,

Alabama



#19 gkillmaster

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Posted 21 April 2014 - 01:40 PM

Thanks Alabama! Great to get your thoughts and input. Its useful information and will help put things in a more meaningful perspective. Much appreciated... You've got me thinking now... :)



#20 alabama

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Posted 11 May 2014 - 11:27 AM

Post Script (p.s.)

 

Twenty years ago, a college professor told me that she had gone to a conference and one of the papers she sat in on concerned burnishing.

Basically, all burnishing does is push temper back in to the clay body and align the clay molecules in one direction, creating a shiny surface.

So when the temps start rising about 1800 degrees (bisque) the outside molecules start burning away, leaving a less than shiny, smooth surface.

When I experimented with burnishing, I didn't apply a slip on the vessels, just burnished the exterior with something harder than the leather hard

clay....Pecan, spoon, burnishing stone, etc.  (I didn't like the back of a spoon)  To achieve a really glossy shine coat the area with some type of oil..

cooking oil is ok...  To get a really really glossy shine, polish the exterior with car wax after firing.  If you want a shiny BLACK finish, cover the vessel

with DRY material between the temps of 850 and 950 degrees.  Do not use material that you THINK is dry... the minimal moisture will cool the vessel down so quick the surface becomes mottled grey and black.  Which is fine if that is what you're going for.  Dry cow, horse, sheep manure is fine as well as leaves...

Leaves catch fire so you may need a 5 gallon bucket of dry sand to smother the leaves.  Pine straw will work but may leave resin residue.  Grass clippings

works also.  But beware that organic material ignite at 451 degrees and the vessels are 950 degrees, so expect the material to burst into flames.  You might want to consider a smaller controllable fire than a large conflagration..ie the greater the number of pots the bigger the fire.  By the way, taking a page out of Tylers post about experience, when your jeans catch fire, it tends to burn up your leg... so sit down to put out your leg.  Explaining how you burned you hands

putting out your legs, doesn't instill confidence.

Experiment!

 

Good luck,

Alabama






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