I recently did a bisque firing and had problems with cracking of my small storage jars. They didn't actually fall apart, but cracks are clearly visible.

I think the problem was I tried to fire too fast, my studio is pretty damp at this time of year and I stupidly tried to rush the kiln because I wanted to go to bed.

The only reason I'm posting this is that the only jar that didn't crack was one that I fired the lid and jar separately, which made me wonder if this is a better way of doing it. I've also thought maybe turning the lid inside out, and resting it on top might be better as there would be less density of clay where the gallery and lid meet...

I'm sure most people fire their jars with lids in place, but I just thought I'd check if this is common practice?

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Tom - I do not bisque-fire or glaze-fire jars with the lids on.  

1.) The lid traps moisture released around 1,150 in the bisque;

2.) The lid adds extra weight and stress to the form below.

3.) Many clays become plastic enough or have enough flux to permanently attach the lid to the jar unless it is first dusted with alumina hydrate or less effective substitutes like kaolin or silica.

Many years ago a ceramics teacher told someone at our studio that he should always fire pots with the lid on to help "assure the final fit".  As they frequently repeat this received wisdom to me, I realize that there are some out there who believe the lid should always be on the pot during a firing, but I have never found this to be helpful.

I think the flat base of a lid is best fired on a flat shelf. Stability in a very plastic body like vitrified porcelain depends greatly on the shape, such as a flange on the neck. But I don't see how it would benefit from the weight of the lid on this flange.

I would like to hear from others why it might be helpful to fire a jar with the lid attached.

Did this ceramics teacher actually say a jar should be fired with the on, and if so, why?

Thanks Norm,

I think the 'assure the final fit' idea was the reason I had in mind when deciding to fire with lids on.

Obviously it saves a lot of kiln space to fire with them on, and I was planning to glaze fire with them on, but I guess there's no point saving space if they end up in pieces.

Would you not fire anything with enclosed space? I did some bowls in the same bisque, fired them rim to rim, they came out ok but I guess it's a similar deal with trapping moisture...

I honestly don't understand the "assured fit" concept of firing things with lids. If a piece and its lid warps and is no longer symmetrical, the lid will only fit the jar in that one particular orientation so what got preserved?

Gravity holds a lid flat onto a kiln shelf just as easily as it holds a lid flat onto a jar - so I don't understand the concept somehow.

The only problems I've seen occur with stacking is when the base of an upper bowl sits too far up the rim of the lower bowl.

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