Jump to content
Sign in to follow this  
LeeU

Preheat Time For Single Fire Cone 6 (Tonight Or Very Early Tomorrow Morning!)

Recommended Posts

Steven Hill recommends a preheat time of 1 to 3 hours, depending on state of the greenware. Someone practiced in single fire mentioned that 10 minutes (rather than the 5 minutes the manual recommends) is sufficient. Big difference! The ware is as dry as it's gonna get-all cone 6. I am doing a Slow Glaze in the L&L with a programmed extended cool down. The thickest pieces are 1/2 in small cut slabs-flat with some shallow surface wells. Others pieces are quite thin, and just a few small bowls. And my Spring Planter for the Community Challenge. Any feedback on how long  I should preheat? Thanks in advance! 

Celia UK likes this

Share this post


Link to post
Share on other sites

On most loads I water-smoke for 1hr. However, with the humidity of the last few weeks, I will fire next load with 2hr water-smoke. I'm not a big fan of fast fire at lower temp, as I have seen too many pieces in puzzle bits at the end of a bisque not done well.

Share this post


Link to post
Share on other sites

Thanks-my gut was telling me a couple of hours would be the smart move. These are mostly test ideas so not a tragedy the single fire doesn't work out the first time. 

Share this post


Link to post
Share on other sites

 using a normal pre-set program called slow glaze on the L&L allows you to add the preheat first.  the preheat takes a very long time, (2hours?) to get to 212 degrees.  a ten minute hold time once it gets to 212 is sufficient.  that is what i told you and why you remember ten minutes.

 

 check your manual for the model you have to see how long yours takes to preheat.

Pres likes this

Share this post


Link to post
Share on other sites

I don't get the idea of pre-heating? Is this only if the ware isn't completely dry? Doesn't a slow first ramp, with bungs out until there's no condensation on the mirror, cover this? Newbie questions again!!! I'm in (sunny!) England, so humidity isn't an issue and I don't bisque fire until things are completely dry.

Celia

Share this post


Link to post
Share on other sites

I never did the mirror check, sounds like a good idea when doing loads with dampness. I did an alternative-placing my hand in front of the bung hole, and feeling the air coming out. If it was anywhere near damp I could tell and leave the kiln until air was dry coming out. When firing student pots that varied in thickness so much it was always better to be safe than sorry. When firing my own work the same follows, but my work is very consistent in thickness, and dries evenly with the strategies I take for drying.

 

best,

Pres

Share this post


Link to post
Share on other sites

celia, we are talking about single firing.  no bisque.  the preheat simply guarantees that the work is completely dry before heating it to glaze temperature.

 

and to really blow your mind, this allows me to mix lots of things in a firing.  there might be a finished, glazed but unsatisfactory for some reason, pot at the bottom, right next to a greenware with a coat of glaze piece, a bisqued piece from a previous firing also glazed for this firing.  i do this kind of thing often.  rarely have problems of any kind.  

LeeU likes this

Share this post


Link to post
Share on other sites

As I thought then - thanks for the confirmation Joel, but 'under 100oC for a while' could mean lots of things! My first ramp is generally 100oC up to 525oC (I'm sure there was some significance in this - is it the chemically bound water thing? I THINK it was more important to leave out the bungs for glaze firing - ?can spoil the glaze, but am not certain. I do understand about getting the moisture out of greenware in the bisque firing, too.)

Old Lady - I've mixed bisque and glaze firings too - when the temps work out, and have also never had a problem. However my glazing is pretty basic stuff - no special effects or the like.

Pres - the mirror works perfectly for me and I've been surprised by how hot the kiln can be and still have moisture coming off what I'd have said was bone dry ware! At 525oC (977oF) there's never moisture, but I've still seen it at 400oC. I find the mirror reassuring as its so visual - guess you just have sensitive hands!!

Share this post


Link to post
Share on other sites

The 525 degree mark is one of the critical quartz inversion points for ceramics. (There are more for glass). This means that the crystalline structure of free silica (silica that hasn't bonded with other oxides in your clay or glaze) changes from one shape to another, and when cooling it changes back again. Because the crystal structure changes shape, it causes expansion upon heating and contraction upon cooling. If the entire piece isn't heated (or cooled) evenly it creates stresses in the structure, and potentially cracking or dunting. This is why pit firers experience such grief if they don't bisque first. They can't go through quartz inversion with an open flame as evenly you can with a well insulated kiln, and there's more unbonded silica in the mix before bisque than after.

The chemical water burning off is definitely a factor, but the thermal expansion of the various shapes of crystalline quartz are a significant force to be reckoned with. Slow and even is your friend.

Edit: Pres is quite right about the temperatures in his post below. The first ramp Celia refers to that stops at 525 C, somewhat before Quartz inversion, to allow for evening out of the heat in the kiln, either through slowing the firing rate or adding a short hold.

Min likes this

Share this post


Link to post
Share on other sites

573C. to 600C. is considered crucial or  1063F. to 1200F. all in all, after you burn out your water, your organics, and then reach quartz inversion, things are pretty easy. I usually shoot to ^06 quickly after red orange..

 

If a glaze firing I shoot up fast after running slow to red heat slowing down at bright yellow orange heat, cresting over ^6 slowly and then back off till yellow orange, hold 15 minutes then drop to red heat and hold for 20 more, and shut down. Don't ask me the cones, or the rate, I do it by time and temp color.

 

Best,

Pres

Share this post


Link to post
Share on other sites

Thanks Diesel - this must be what I'd found in my research when I was trying to resolve my glazing issues, but I'm thinking I got 525 & 575 confused somehow. I got into such a stress with glazes crazing and cracking a couple of years back, that I bought a digital controller and came up with a program that went carefully through ALL the 'dodgy' phases incorporating every bit of advice I found. This included a slow start, slow through Quartz inversion, slow for the last 150o, a hold, AND firing down! However, this means that I've never really identified which thing resolved the problem! Should I go slowly from about 570 to 600oC then and the same on the way down through Quartz inversion? I can feel a bit more brain work coming on! At the time I was happy to have got rid of the crazing but I knew I'd over-complicated the firing program so I've simplified it a bit now and will review again.

 

Such an intuitive potter Pres - no doubt from years of experience! Sounds like a lot of babysitting the kiln. I'm now going to match up your colours with temps on my chart and see how my firing program matches up.

 

Thanks all for sharing your expertise.

C

dolly likes this

Share this post


Link to post
Share on other sites

Opened and unloaded today...the cone 6 single fire came out great!

 

I did not do the controlled cool (need to get a better handle on programming that-it was just too complicated along with the delay and the preheat...should have done the whole programming over, but I did not feel well/in pain--surgery tomorrow-and just did not have it in me. A little bloat on the black clay and a righteous S crack appeared in the disk of a thrown candle holder, but other than that, perfect. Here's a pic of the top cone pack & the basaltic body. Thanks especially Old Lady & Pres. I was almost out of time when you responded because my wall piece for a local exhibit was in there that must be delivered before I go to the hospital tomorrow, so I was able to get it done in time to do the finishing embellishment today. 

post-63409-0-57166800-1434335768_thumb.jpg

Share this post


Link to post
Share on other sites

humidity where i live is usually in the very uncomfortable range, the dew points often way over 70.  glazing is done on the same day i put pots into the kiln so i take that precaution of preheating since i single fire.  do not regret it.

Pres likes this

Share this post


Link to post
Share on other sites

A question for those of you who single fire. Do you dip your entire pot in one glaze? I tried to single fire once, and I dipped the top quarter of the pot in one glaze,

and when I could handle it and went to dip the bottom 3/4, it just broke right off at the edge of the first dip. When I went to dip a whole pot in glaze, it fell apart. So, what do you do?

Share this post


Link to post
Share on other sites

there are potters who can dip greenware but it is not something i have ever done.  i use a sprayer.  i do not thin the glaze at all.  just turn up the pressure so that the normal thickness of glaze is pushed out in a fine spray all over the piece.  it dries immediately and can be handled right away.  you have to realize that most of the things i make are flat and rarely do i throw any more. look at the things in my gallery.   i love to make bowls and they are sprayed, too.    

 

occasionally a particular glaze may have more water in it than it should.  if i spray it and it drips, i know it is too thin.  so i do another color and let the first one evaporate or scoop some of the water off after it settles overnight.  watery glaze does not coat the pot enough and makes it too wet.

 

maybe a tiny experiment would help.  roll a piece of your usual clay out flat about 1/4 inch thick.  let it dry totally.  then dip it halfway into water for only a second or so.  break the wet part and notice how little of the clay is actually wet.  i find that people say they cannot trim their pots because they forgot to cover them and the pots are totally dry.  well, as long as you have a damp sponge (on the wet side of damp), you can trim a dry greenware pot.  it just takes a few minutes longer to wet the clay enough to cut some away a little at a time.  cut and wet, cut and wet, and it is done.

Pres, ShellS and Karen B like this

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

×