Pinholes using SLOW bisque fire schedule and vent-sure

[FGC]

Hello, 

I'm hoping I can find some help with this topic.  I have a new L&L JD230-3 kiln with a vent-sure.  I've been using Laguna WC-609/#65 clay body.  I use a slow firing schedule by Steve Davis that I read about in the July/Aug 2018 issue Pottery Making Illustrated.  It is about a 15 hour schedule where the ramp is 60 degrees/ hour from 1300 deg to 1650 degrees.  My schedule is even slower than the published version.

My ware is small, thin thrown and handbuilt pieces and is spaced quite evenly, no stacking in the bisque fire. I have the required two holes drilled in the bottom slab for the vent.  I'm getting pinholes using various Mayco Stoneware, and Stroke & Coat glazes and and Amaco Celadon and Potter's Choice glazes.  I've called Mayco who thinks it may be better to use Bmix or a different clay body.  I've called L&L's distributor, who said I could drill a 3rd hole in the bottom but wasn't sure that would do anything and of course does not recommend drilling holes in the top.

These are the other things I've tried:

Since all of these glazes are brushed on, using a fan brush, layering thick, layering thin, bisque firing to a hot 03, dunking the bisque ware in water, wiping it, allowing it the water to absorb and glazing. Doing a 180 degree drop from peak temp and hold for 1 hour during the glaze firing and let kiln cool the rest as normal.  Doing a drop from peak at cone 6 temp down to 1950 and holding for 30 minutes then let the kiln cool naturally.

My thought is that I'm just not getting enough clean air into the kiln to oxidize the gasses.  The slider on the bypass box was set to less than 1/3 open.  I intend to close it more  to leave only about 1/8" open. 

Does anyone have any experience using an L&L kiln and vent-sure that doesn't seem to vent adequately?  I'm wondering about drilling holes in the lid. How can i safely get more fresh air into my kiln?

Also, does anyone know if Laguna has a cleaner burning white cone 5/6 clay than #65 that is smooth and can be used for handbuilding?   Or has anyone seen a difference in the pinholes between Bmix5  and #65?  It seems the clay shouldn't be that gassy.  But if anyone has experience on a 'cleaner' burning clay, I'd love to hear from you.  I've called Laguna, but haven't heard back from them.

 Thank you for your help!

Suz

 

[liambesaw]

I don't see much in the msds that would make me think there's a ton of things to offgas in that clay body.

I will say that amaco Celadon didn't pinhole for me on bmix.

Edit: it has less than 2% dolomite, which is around 50% co2.  Other than that the other ingredients are for the most part low LOI. Could that 2% of co2 be causing problems?  It's possible!  That means that 1% of the entire clay body is co2

[Bill Kielb]

2 hours ago, FGC said:

My thought is that I'm just not getting enough clean air into the kiln to oxidize the gasses.  The slider on the bypass box was set to less than 1/3 open.  I intend to close it more  to leave only about 1/8" open. 

Does anyone have any experience using an L&L kiln and vent-sure that doesn't seem to vent adequately?  I'm wondering about drilling holes in the lid. How can i safely get more fresh air into my kiln?

Also, does anyone know if Laguna has a cleaner burning white cone 5/6 clay than #65 that is smooth and can be used for handbuilding?   Or has anyone seen a difference in the pinholes between Bmix5  and #65?  It seems the clay shouldn't be that gassy.  But if anyone has experience on a 'cleaner' burning clay, I'd love to hear from you.  I've called Laguna, but haven't heard back from them.

 Thank you for your help!

Suz

 

It really does not sound like a vent issue. Closing the slider excessively is also not the best idea as the bypass air really cools off the blower and extends the life of the blower and ductwork. The smoke test in their instructions done at startup and 100 degrees is probably your best method of tweaking and adjustment. If I could not get some suction at 100 degrees and the bypass half open using the smoke test then I would contemplate drilling the extra hole. My goal would be to get slight suction and still have the bypass open enough so my exhaust temperature stayed below 130 degrees or so. 

The gassy part is difficult also as the majority of everything off gassing should leave in the bisque firing. My experience with vent sure  is it performs well. Is the collection box mounting reasonably air tight  and the discharge free of obstruction would be my only  other question if not able to perform the smoke test successfully.

Pinholes

do you have some pictures of the pinholes inside and out?

can you run a quick test with Bmix and your clay - maybe a small bowl for each, not just a test tile with something that for sure pin holes?

Do the  pinholes appear more on the first glazed surface or the inside or outside?

what is your glaze firing schedule?

pinholes can be a pain to diagnose but in my experience not necessarily the result of kiln ventilation 

what are you firing to and do you confirm with cones?

we did a kiln exhaust redesign and you may find some of the measurements interesting. We are really talking small quantities of air, especially actual kiln air in a properly operating system. The exhaust seems unlikely the cause (in my view) if it working per the instructions.

 

 

[GEP]

I agree that this is probably not a venting issue. And that I would like to see photos of your pinholes. Can you post some photos? 

[FGC]

Thank you for your reply,

To answer your questions,

I will get some pictures of the pinholes uploaded later today.  I do not have Bmx5 clay at this time to run tests. 

The pinholes seem arbitrary, some glazes like Cinnabar from Mayco will have only one pinhole in a piece and others like Blue Surf will have multiple scattered throughout.  When applying the glazes, I do not shake the containers--only stir the glazes.  I notice there are always bubbles present while brushing the glazes.  My work is quite textured using bisque stamps. However, the pin holes appear on the inner smooth surfaces as well as on the textured surfaces.   As I apply the glaze I load the brush well and go slowly to prevent more bubbles from appearing.  After each coat (I apply 2-3 coats depending on the recommendation on the container) I inspect the piece for pinholes and rub them out with my finger or a pointed wooden tool.  As each piece is finished and ready for firing I inspect again for pinholes and rub/clean to erase.  So at the time of loading there are no visible bubbles in the dried glaze.

The firing schedules I've used are  medium slow to cone 5 or cone 6, The cone 5 schedule was about 10 hours with a drop and soak for one hour at 2012 degrees.  The cone 6 schedule was 12 hours with a drop and soak at 1950 degrees for 30 minutes.   I use witness cones toward the center of each shelf.  In both cases the cones were touching the shelf so it over-fired slightly.  

I noticed, using lit incense smoke a lot of room air leaking into the joint at the bottom of the fan assembly where the 3" pipe attaches to it.  I unscrewed it to see if there was some way to close the gap but there is not.  So it is pulling room air into that area before it exits the building.  Again, I'll post a photo.

Suz

[FGC]

The smoke is being pulled into the fan motor assembly, see photo 1, and the gap between the 3" tube and the collection box that is welded to the rolling kiln stand under the kiln.  But at 100 degrees the smoke is not pulled into the kiln near the lid or any other place around the body of the kiln.

[Bill Kielb]

Interesting! The sheet metal transition  gaps are (unfortunately) typical average construction and likely will not affect the system performance. Less than 1 CFM (cubic feet per minute) of actual kiln air is drawn out through the tiny drilled holes and the rest is makeup room air from the slot and from leakage into the piping. The blowers typically are rated more than 100 CFM  and require lots of excess room air to cool their own motors so this leakage while not great is probably fine, and helping to cool the blower motor as it runs and generates heat. The smoke test suggested in the instruction manual, (cold kiln) is still probably your best indicator if it is functioning as designed. Even with this leakage the negative pressure in the manifold that attaches to the kiln only requires about 0.1” of negative (suction) pressure to perform adequately. If you are super curios I can show you how to build a simple manometer for nearly nothing to check this.

This fit can be sealed using most duct sealants and I also notice a split in the seam of the hard pipe that likely leaks a bit. So sealing all these will increase your suction but again the blower needs enough air to cool itself. I go back to adding a hole in the kiln and ensuring the air in the exhaust does not get hotter than 130 degrees when the kiln is at top temperature and the fan is running. I believe the instructions call for slight suction then back off to neutral.

The stain - if bisque seems to be off gassing from a pot and even if glaze, same issue.  Since it is not under the foot contact area but everywhere else I would say contaminants burned out of the clay but can also happen with glaze colorants if you glaze the bottom inside foot ring of your pots.

Pinholes- these are interesting in that they appear to extend to the body and actually many may appear to have healed which starts to make me think this is more of a packed bed porosity, coalescence of bubbles and a glaze that does not heal all that well because of viscosity and surface tension issue especially where the bubbles have combined and grow  large. Often when I get glaze and clay combinations that are this difficult I abandon them if I have to always do too much to get repeatable success.

Anything  else you can tell us or show us would be great. Did your holds improve the situation, make it worse, or no effect? Lots of expertise and experience on this forum!

I think from what I see I would suggest testing for a packed bed porosity issue which usually starts with bisque to a lower temperature, maybe 05,06  progress through glazing  inside first, wait a day to dry, then outside and finally confirmed by adding one to five drops of Darvan at the time of glaze to see if  we can displace enough air to solve the issue.

Just some thoughts for now

[FGC]

I did the smoke test with a cold kiln and heated to 150 degrees to make up for 43 degree TC offsets and no smoke was going in around the lid or kiln.  Slider on bypass box was closed.

 Yes, I'd love to know how to make a manometer! 

This piece was from a drop and hold from a slow cone 6 down to 1950 degrees for 30 minutes.  I've never heard of the term 'packed bed porosity', it sounds like you're saying the COE of the glaze is too tight for the body?  And the viscosity, (the brushing medium the mfr added won't allow it to flow?) This clay, #65 has only a 4.8 COE and is crazing with other glazes.  I did bisque this piece to 03, I read on Digital Fire to try bisque firing hotter to get rid of pinholes.  

Thank you for your reply!  

[Bill Kielb]

Stubborn pinholes!

The packed bed porosity issue would be the result of some glazes not being able to get into the pores of the bisque at the time of glazing so there is a log jamb and trapped air. Bisqueing to a lower temperature allows the glaze to infill more effectively and basically be absorbed more completely. Since trapped air needs to come out by way of buoyancy it must float up through the glaze. If several bubbles combine to form a bigger bubble it can exceed the glazes healing ability even with a drop and hold so random pinholes can be the end result with many healing but the larger ones unable to heal. I would test for it if I really wanted to keep using these combinations.

[Magnolia Mud Research]

Try pre-wetting the bisque before applying the glaze. 

 

[Babs]

Is the halo on a kiln shelf or pot?

Looks like something fuming onto shelf from glaze on pot.  White cirxle pot footring

[GEP]

20 hours ago, FGC said:

In both cases the cones were touching the shelf so it over-fired slightly. 

This is another area worth exploring. Overfiring some glaze/clay combinations by just a little can cause pinholes. I developed a pinholing problem when I switched claybodies years ago, The glaze worked fine on my old claybody, but pinholed on the new one. I tried a bunch of possible fixes, and the answer turned out to be to lower my firing temp from cone 6 to cone 5.75. 

[neilestrick]

If you're not getting fume odors during the firing, then the vent is working fine. It can be very difficult to do the smoke test through lid gaps. Keep the slider at 1/2 unless you're getting odors. During wax burnout at around 500F you'll usually get some odors no matter what, but beyond that if there's not smell, the vent is working and you're getting plenty of air. That vent motor is more than strong enough to overcome all the little gaps in the system. Make sure the holes in the kilns floor are not obstructed.

[Bill Kielb]

On 8/11/2019 at 9:08 PM, FGC said:

Yes, I'd love to know how to make a manometer! 

Thank you for your reply!  

@FGC

Ok, (Overkill for most but you asked)

20 minutes of couch sketching gets you a quick drawing below. Basically having measured these counter flow exhausts  they end up about 0.1” -  0.15” of water column at the collection manifold. Sounds  more complicated than it is. It is simply the small amount of force necessary to lift a column of water up one tenth of an inch. Not very much! So because this is so small the remedy is to take this one inch  vertical column of water, incline it and stretch it out to say 10”. Voila, instantly easier to see a  one tenth inch rise.

In case you are curious, fan pressures are tiny so we often measure them in W.C. (Inches of water column) or inches of h2o. How small is that? Your car tire is probably inflated to 32 psi.  It takes just under 28” of water column to equal 1 psi! So tiny, tiny pressures, especially when we are measuring tenths of inches.

I have watched folks use these temporary manometers  in a pinch and they can be very accurate depending how much care is taken in making them, marking them and leveling them. The manometer must be level when filling and measuring, which also means when constructing really square (90:degree) and parallel construction.

Measure at the collection box where the yellow arrow is in the picture. Simply stick the vinyl hose in the box, no pointing it toward the hose or sealing it in. we simply want the negative pressure within the box. I used to teach HVAC fluid mechanics, etc... and generally when I made the students start thinking of air as a fluid that acts like water and moves from a higher pressure to a lower pressure as opposed to a breeze they understood far better and went on to design ductwork, fan blades, airplane wings ..... you name it.

finally, the instructions from L&L really stresses that the bypass opening  is for fine adjustment and the number of holes drilled and or plugged will have the largest effect. I highlighted the section illustrating this.

In the design I provided, the suction pressure will pull up the water and you will read the suction pressure as the water is drawn up the tube. All connections need to be reasonably tight but this is fairly easy with the vinyl tubing if the hole is drilled slightly undersized from the tubing and the tube is angle cut before inserting it into the hole.

good luck, hope that helps

 

[dhPotter]

Not sure if this will be relevant to your case - I have been using Laguna 609 #65 for about 3 years now. BUT, I single fire and use my own glazes and do not get pinholes. My firing schedule goes slow in the first 500* because of single firing. Then goes at 400* per hour. At 2050*F the ramp slows to 108* per hour till 2185* for a 15 minute hold. Then free fall to 2085* for a 20 minute hold. The changing to 2050*F to a slower speed going up and the hold at 100* less than peak temp on the way down are supposed to get rid of any pinholes. Again I do not have pinholes. Try single firing. I use glazes found in John Britt's cone 6 glaze book and from Mastering Cone 6 Glazes book.

Glaze cone 6...

Segment        Rate F*/HR    Temp    Hold
   1            200         220     30-60
   2            100         500      0
   3            400        2050      0
   4            108        2185^     15
   5           9999        2085     20
   6           9999        1700      0
   7             50        1600     60
   8             50        1500      0

[Roberta12]

You could try lowering your bisque temp.  And as Mea said, possibly your glaze temp.  Clay/glaze seem to be one big research project.  I had a glaze that worked well on one clay body and pitted horribly on another.  I am going through a bit of crazing right now, because I tried a different clay with the clear glaze that I mix.  It's making me crazed!

But I don't think it's your kiln.   

Roberta

[FGC]

8 hours ago, dhPotter said:

Not sure if this will be relevant to your case - I have been using Laguna 609 #65 for about 3 years now. BUT, I single fire and use my own glazes and do not get pinholes. My firing schedule goes slow in the first 500* because of single firing. Then goes at 400* per hour. At 2050*F the ramp slows to 108* per hour till 2185* for a 15 minute hold. Then free fall to 2085* for a 20 minute hold. The changing to 2050*F to a slower speed going up and the hold at 100* less than peak temp on the way down are supposed to get rid of any pinholes. Again I do not have pinholes. Try single firing. I use glazes found in John Britt's cone 6 glaze book and from Mastering Cone 6 Glazes book.

Glaze cone 6...

Segment        Rate F*/HR    Temp    Hold
   1            200         220     30-60
   2            100         500      0
   3            400        2050      0
   4            108        2185^     15
   5           9999        2085     20
   6           9999        1700      0
   7             50        1600     60
   8             50        1500      0

 

Wow! Your firing schedule just blew me away--and my theory of the vent not working up to par.  After reading your post, the pinholes in my work seem to point to a unhappy clay body/glaze fit.  I've grown to really enjoy WC609 and hate to change and just recently purchased many commercial glazes.  

Is your pottery mostly thrown and thin?  I hand build with slabs no thicker than 1/4"  with this clay.  What kind of kiln/vent set up do you use?  And do you turn the fans off for the cool down?

I haven't ventured to dive back into glaze formulation,  I took it many years ago in college.  I'm glad you referenced the books you used.  I notice that this clay also has a slightly lower COE than most of the other popular Laguna clays.  Have you had any difficulty with crazing?

All that you can show and would like to tell about your work and glaze recipes I'd love to know.  

Thank you again for blowing my socks off with your ONCE--I feel like I'd be teetering on cliff--fired schedule!

[FGC]

20 hours ago, Bill Kielb said:

@FGC

Ok, (Overkill for most but you asked)

20 minutes of couch sketching gets you a quick drawing below. Basically having measured these counter flow exhausts  they end up about 0.1” -  0.15” of water column at the collection manifold. Sounds  more complicated than it is. It is simply the small amount of force necessary to lift a column of water up one tenth of an inch. Not very much! So because this is so small the remedy is to take this one inch  vertical column of water, incline it and stretch it out to say 10”. Voila, instantly easier to see a  one tenth inch rise.

In case you are curious, fan pressures are tiny so we often measure them in W.C. (Inches of water column) or inches of h2o. How small is that? Your car tire is probably inflated to 32 psi.  It takes just under 28” of water column to equal 1 psi! So tiny, tiny pressures, especially when we are measuring tenths of inches.

I have watched folks use these temporary manometers  in a pinch and they can be very accurate depending how much care is taken in making them, marking them and leveling them. The manometer must be level when filling and measuring, which also means when constructing really square (90:degree) and parallel construction.

Measure at the collection box where the yellow arrow is in the picture. Simply stick the vinyl hose in the box, no pointing it toward the hose or sealing it in. we simply want the negative pressure within the box. I used to teach HVAC fluid mechanics, etc... and generally when I made the students start thinking of air as a fluid that acts like water and moves from a higher pressure to a lower pressure as opposed to a breeze they understood far better and went on to design ductwork, fan blades, airplane wings ..... you name it.

finally, the instructions from L&L really stresses that the bypass opening  is for fine adjustment and the number of holes drilled and or plugged will have the largest effect. I highlighted the section illustrating this.

In the design I provided, the suction pressure will pull up the water and you will read the suction pressure as the water is drawn up the tube. All connections need to be reasonably tight but this is fairly easy with the vinyl tubing if the hole is drilled slightly undersized from the tubing and the tube is angle cut before inserting it into the hole.

good luck, hope that helps

 

I have a few questions about how to construct this nifty tool.  I'm not sure what Bottle Rx film means, is that a glass bottle? How big?

I see an angle on the 10" tube leading from the bottle to the 90 turn, what is the angle? Or is it important?  What is the length of the tube sticking up on the right side of the picture?   Do I use an tube 'elbow' to make that 90 turn?  

Lastly, is this whole apparatus sitting inside of  a hollow wood or foam box?

You didn't know you'd open a can of worms here did you?  

I read that part about gross adjustment, I just don't know if it voids the warranty on the kiln if I were to drill a hole in the top.

Thank you again,

Suz

[Bill Kielb]

Bottle is small plastic  bottle, maybe a prescription bottle. Angle not important, you will be stretching one inch vertically across ten inches. Draw a rectangle 1” high by 10” long and run your tubing from lower left corner to upper right and hot glue in place. No elbows necessary just bend the tube up  neatly and hot glue in place. This all can be mounted on a foam card or,piece of wood, anything with some substance that can stand up or be binder clipped level while you fill and use it.

the tube on the right is left open to the air and can terminate just above. maybe 1”. It only lets air in so not that important. The tube on the left can be as long as you like it is your working tube or measuring end.

last thing, I doubt you need to drill holes in the top, there are plenty of openings in the lid and section seams that likely will easily leak air inward.  Now adding another hole in the bottom, maybe if needed will have a big effect.

here is a picture of what you will be sort of be creating but a manufactured model. Yours is the home hack for a few dollars 

 

[dhPotter]

Orton VentMaster is the vent. The vent stays on till I open the kiln to remove wares at around 150*F - 250*F. Remember, Starting at segment 3, the firing schedule is just another glaze fire schedule. The down firing and holding is to encourage crystal growth. The schedule is basically Stephen Hill's firing schedule. Panama Blue, Selsor Chun  and Kitten's Clear come to mind as crazing. I don't glaze calculate - I use recipes from those books and Tony Hansen's website. I do have and use DigitalFire glaze software. Take a look at the Evolution album in my gallery. In some of the comments I mention what glazes are being used.