Covered jars

[CactusPots]

I want a sour dough crock, so I'm making a series of covered jars.  Never make one of anything is my mantra.  I don't have a market for stuff like this, but don't seem to have trouble giving it away.

I guess that the prevailing wisdom is to wax with alumina the mating surface of the jar and the lid so to fire it in place.  This requires confidence the glaze does not run.  

Does anyone use any other method?  Fire the lid separately flat on a shelf, maybe?  I like this since it will make the post height requirement less.

Pros and Cons?

[Min]

For everything except porcelain I don't find it necessary to use alumina wax, just the wax resist is enough. You are going to get a better fit if the lid is fired on the pot, they tend to go out of round a bit if fired on a shelf. If you do decide to fire the pieces separately then use a clay donut under the lid so it can shrink along with the lid rather than the lid getting hung up on the kiln shelf. 

[JohnnyK]

A lot depends on the type of lid that you make and how it fits the jar.  You probably know the shrinkage rate of your clay so if you make a lid that sits atop the jar instead of in a gallery, just make sure the underside of the lid, if it protrudes down, has enough slop to fit inside the jar after it is fired...

[liambesaw]

If I don't use alumina wax, my red clay will flux together too. I just hit it with a torch for a minute and that seems to do the trick.  Every time I've tried firing separately the lid goes oval

[CactusPots]

No reason not to use alumina on direct clay to clay contact that I know.

I usually make an L shape top rim in the can and a dome lid with a flat rim.  So the contact is flat to flat inside the rim of the pot.  I throw as close as I can and still be over, then trim to final tight (I hope) fit.  I haven't made enough covered pots to experiment with a lot of variations.  I know from my library that there are other possibilities.

[liambesaw]

24 minutes ago, CactusPots said:

No reason not to use alumina on direct clay to clay contact that I know.

I usually make an L shape top rim in the can and a dome lid with a flat rim.  So the contact is flat to flat inside the rim of the pot.  I throw as close as I can and still be over, then trim to final tight (I hope) fit.  I haven't made enough covered pots to experiment with a lot of variations.  I know from my library that there are other possibilities.

The only reason not to is that if you get a little too much wax somewhere it'll move and the alumina can make surrounding glazes look gross.  I've had this happen with surprisingly little wax.

One nice thing about the alumina wax though is after it's fired, there's a layer of alumina grit that you can use to true the lid to the pot just by getting it wet and rotating it back and forth

[Babs]

Sometimes lids do stick a bit on firing in situ.

I find if I hold onto lid and rap ithe pot body sharply with a wooden stick or spoon they separate.

(Do this just a short distance above your table:-//)

[Min]

3 hours ago, CactusPots said:

No reason not to use alumina on direct clay to clay contact that I know.

It's the accidental alumina hydrate to glaze thats a problem, unless you are trying to make sandpaper of course. (could be a thing for cactus pots?) Great stuff for stopping clay to clay fusing.

[neilestrick]

Depending on the type of pot and lid, you run the risk of either the pot or the lid warping slightly during the glaze firing if they are fired separately. If they fit well before firing, it will only take a tiny bit of movement to mess up the fit. There are ways of dealing with runny glazes via application, or you can choose a type of lid where it's not really an issue. The glaze combination on the pot below is incredibly fluid, but because only a small amount is on the underside of the lid, and it's not vertical there, it won't run into the jar. The other nice thing about this type of lid is that the unglazed portion doesn't show when the lid is on.

 

 

 

[Babs]

Snug and beautiful Neil!

[Callie Beller Diesel]

Lidded jars are one of my favourite things to make!

Everyone else has already chimed in with the good stuff. I am in the no alumina camp, even though my clay does tend to stick to itself. I have a stick next to the kiln, and a few firm taps frees everything up. I always fire the pieces together because I overfire my clay and it moves. 
 

I mostly do a lid that sits in a gallery. I find if you throw the pots and the lids at the same time (no shrinkage happening before you take  the lid measurements), the lid will shrink a hair more than the gallery will, because the clay has a tendency to cup back in. This happens both in drying and in the glaze. If you make your lid carefully to the exact measurement of the gallery, the shrinkage will create the amount of room you need to have the lid fit tightly once the piece is finished. I find if the lid fits perfectly in the green ware stage, the lid will be too big in the end piece. 

[neilestrick]

Thanks @Babs !

[CactusPots]

2 covered jars in this firing.  One made it,one didn't.    Couldn't get one lid unstuck with heat and gentle taps.  Broke the pot with more aggressive approach.

I think for dark clay the alumina will be my choice in the future.

[Mark C.]

8 hours ago, CactusPots said:

2 covered jars in this firing.  One made it,one didn't.    Couldn't get one lid unstuck with heat and gentle taps.  Broke the pot with more aggressive approach.

I think for dark clay the alumina will be my choice in the future.

Alumina is your friend-when they really stick I used 2 inch diameter 1.5 foot long piece of bamboo to tap the pot not the lid lip much. I tap all around for a long time patently. If that does not do it I put the jar I'm chest freezer for a few hours thereunder a water stream only on the pot (not the cold lid I make the stream really hot and the clay expands and the cold lid does not(this is for the kind of lid like Niels shows in his photo above. If you have the other kind of lid leave the pot cold and hot water on just the lid. It usually always works -never lost a pot this way. But occasionally I have had to freeze them twice.The idea is expand the outer layer while leaving the inner frozen and smaller.One word of caution is the lid can literally pop off so be ready.

[Pam Pulley]

I have seen using small bits of wadding glued in place under the lid at the lip of the piece of work.  Works really well.