Glaze reclamation

[Simon Heath]

Hi there,

I'm hoping for some advice.  I recently mixed up a 5 kg batch of glaze.  Lots of pinholes.  I just realized the 1.8% Manganese Carbonate I used wasn't Manganese Carbonate.  I don't know what I used - fluffy white powder (ha!), maybe Lithium carbonate, but who knows.  Since the glaze contains cobalt carbonate and copper carbonate, and there is  so much of it (too much to fire into bricks), I'm thinking my only choice is to see if the local waste management landfill will take it under their Hazardous Household Waste program.  Any other suggestions?

Thanks in advance,

Simon

[Min]

1 hour ago, Simon Heath said:

I don't know what I used - fluffy white powder (ha!), maybe Lithium carbonate, but who knows. 

Most common mixup with manganese is to use magnesium instead, magnesium carbonate is very light and fluffy so it could be this. Either way, lithium or magnesium carbonate will lower the expansion of the glaze. Did the glaze run excessively?

I wouldn't toss the glaze just yet. Pinholes are often a firing issue, either inadequate burnout during bisque or the glaze firing schedule needs tweaking. I would suggest posting your clay type, bisque firing schedule and glaze firing schedule and glaze recipe before getting rid of the glaze.

[Simon Heath]

Thank you Madeleine!  I would certainly love to save it if I could.  The manganese/magnesium thought makes a lot of sense.  The glaze is Campana Grey (recipe below), pretty well-known glaze, it did run a lot, but the recipe says it tends to anyway so I just assumed it was the recipe.

I have long battled pinholes and seem to have fixed them with all my other glazes - I fast bisque to 04, blast the bisqueware with an air compressor before glazing and have a slow cool firing schedule of: 108 degrees/hr to 220; 350/hr to 2000; 108/hr to cone 6 - 10 min hold; 300/hr to 2100 - 20 min hold; 300/hr to 1900 - 15 min hold; 150/hr to 1400.

I've used it on both Bright White porcelain and Mid Cal 5 stoneware (both Tuckers, Canada), pinholes on both, but worse on the stoneware

Campana Grey

3134 - 21%

EPK - 20%

Silica 20%

Wollastonite 20%

Spodumene 11%

Zinc Oxide 8%

----

Manganese Carb 1.8%

Copper Carb 0.6%

Cobalt Carb 0.15%

 

[Min]

Couple thoughts,

Do you know how much water you added? Just for the sake of explaining my process here I'm going to just use a figure of 80 water per 100 grams of dry base, adjust it to what amount of water you used, use Brongniart's calculator if you aren't sure. (not including colourants) I would take 180 grams of wet glaze add the 1.8 manganese carbonate plus 2 silica plus 2 epk and try that to get the colour right. For the pinholes, does these firing schedules work for your other glazes and do you verify cone 6 with large cones? This glaze is high in fluxes (relative to the silica and alumina amounts), sometimes pinholes and blisters are from overfiring a very fluid (highly fluxed) glaze with high surface. (zinc contributes to surface tension)

BTW are you sure it's manganese carbonate and not manganese dioxide the recipe calls for? I've seen this recipe with both, doubt it's part of the problem though.

[Simon Heath]

Hoo boy - very appreciative of your response, but trying to wrap my head around it!  No idea how much water, though it was 5 kg of dry material to approx. 145 sp gravity, is there any way to calculate based on that?  Do I have to weigh my water?  Rural area with quite hard water.

Just to clarify your suggestion - 180 glaze and only mang. carb, silica and EPK?  Not copper carb and cobalt carb and all the other ingredients?  And does the (probably) magnesium carbonate's effect on the glaze need to be counterbalanced?  I don't use glaze software, so my level of sophistication is somewhat limited.

Yes, I do find the cone pack shows it reaches cone 7 - my other glazes are very diverse (Tenmoku Gold tea dust; a high calcium matte variegated blue from Mastering Cone 6 glazes, 5x20 and a few others - although I did notice some pinholes in a cream rust that I've never had pinholes in before).  My slow cool firing was to try to get rid of pinholes, not create them!  Also, trying to grow crystals in Binger, but that's another story.

Yes, calls for carbonate, not dioxide - I have a different glaze that uses the very black manganese dioxide.

Thank you - you're very generous with your responses.

S.

[High Bridge Pottery]

Min is right, Brongniart's formula is the way to go.

 

Grams of dry material per volume of glaze = (weight of volume of glaze - weight of volume of water) x 5/3

 

As long as you have a way to measure the same volume you can weigh that volume of water, weigh that volume of glaze and then minus the weight of water from the weight of glaze and then multiply by 5/3. Now you know how much dry glaze is in your sample volume to test. Don't try and fix the whole bucket at once.

 

Edited February 24, 2023 by High Bridge Pottery

[Simon Heath]

Okay, thank you, I think I can work this out to calculate dry glaze material in the sample volume.  But is the fix to just add in the Manganese carbonate, more EPK and silica, or does the effect of the magnesium carbonate (I've become convinced that's what I did) need to be offset?

Thank you!

S.

[Jillyissilly88]

I'm sorry to hear that you mixed up a 5 kg batch of glaze with lots of pinholes. It sounds like you have a difficult situation on your hands.

[Min]

1 hour ago, Simon Heath said:

But is the fix to just add in the Manganese carbonate, more EPK and silica, or does the effect of the magnesium carbonate (I've become convinced that's what I did) need to be offset?

I think the firing might be at the root of the problem, could be overfired. Could you post a picture of the pinholes and an image of how much running you are getting, plus a witness cone or two.

Adding the manganese will help with the colour correction. Manganese often lends a purple(ish)brown tone to glazes. EPK and silica will bring the alumina and silica up to where they were in the original formula, would have a minor effect on reducing running but probably negligible. 

Have a look at the charts below, specifically at the alumina (red box) and silica (blue box). I was simply balancing those by adding the epk and silica. You can see that adding magnesium carbonate has very slightly decreased the COE (coefficient of expansion) and increased the LOI (loss on ignition). Both these are fairly minor amounts and I doubt they contributed significantly to your pinhole issue.

I would just try altering a small amount of the base before trying to correct the entire bucket. 

[neilestrick]

Are the hazardous waste regulations so strict that they would care about the 7.5 grams of cobalt in the 5kg?

[Simon Heath]

Thanks everyone.  No, the tiny amounts of cobalt and copper probably aren't an issue, just trying not to dump glazes in my backyard.

Thanks Madeleine.  Despite having read several books on glazes (several times), I still struggle with the chemistry.  I see the silica to alumina ratio has changed slightly along with the COE and LOI, but don't know how to adjust.

The thing is, I got the pinholes on a standard programmed cone 6 slow glaze fire and did the slow cool with holds to try to fix  the problem.  Below are photos from the slow cool firing.  Running isn't a huge problem - two of  the three pots shown are fine.

 

 

[Min]

I would call those blisters inside the bowl more than pinholes. There are a myriad of possibilities for blisters, link here with explanations for many of them. Problem comes from figuring out which explanation makes sense. I'm still thinking the problem is in the firing, looks like cone 7 is down most of the way. I have seen this glaze fired to cone 5 1/2 so perhaps a simple test would be to fire lower and see if that solves the problem. I would still do the soak and drop and hold though I would drop at 9999 (rather than 300F) to 100F below your top temperature then do the hold then slow cool. At the same time I would include a fired pot and see if the blisters get worse on refiring. If you do this then put the pot on a waster slab / cookie to catch glaze runs. If blisters get worse on refiring then it supports the theory of a highly fluid yet viscous glaze melt. Adjusting the drop and hold firing in conjunction with the top firing cone would be how to fix it.

Hansen and Britt did some work with this base glaze, links to their articles here and here. I'm not seeing others reference problems with blisters but they do mention dimples (semi healed pinholes) and clarity of the base glaze.

[Callie Beller Diesel]

I agree entirely with Min about the pinholing issues in this glaze so definitely do the troubleshooting and testing she recommends. 

 But you also mention having struggles with pinholes with more than one clay body and multiple glazes, and you’re using a fast speed on your bisque. Consider slowing your next bisque speed to medium or even slow, and do your glaze cycles as usual.  See if that helps anything.

[Simon Heath]

Thank you again, both!  I'm amazed at the generosity of the people on this forum.

I will drop it to a slow bisque (I had already gone from 06 to 04 to try to address pinholes).  I've just moved from a manual kiln sitter to programmable, so still figuring it out. I originally dropped it at 9999/hr, but got an error code for it being above the target temperature and was told to go for a  less aggressive rate of cooling.  So, that's why I'm dropping at 300/hr, which hasn't triggered the error.  Dumb question - is the only way to fire to cone 5 1/2 to manually enter the target temperature?  In my current firing cycle, I program peak temperature as cone 6 (and yes, it definitely looks like cone 7, though that is the top shelf of the kiln).

Thank you for the links on Campana - the grey definitely looks much greyer than mine.  I might actually try the recipe without the manganese, just the small amounts of cobalt and copper (assuming I ever get this existing bucket of glaze sorted).

S.

[Min]

7 hours ago, Simon Heath said:

is the only way to fire to cone 5 1/2 to manually enter the target temperature?  In my current firing cycle, I program peak temperature as cone 6 (and yes, it definitely looks like cone 7, though that is the top shelf of the kiln).

You could set it cone 5 then add a soak to get to cone 5 1/2. If you are currently firing to the preprogrammed cone 6 fire and it's bringing cone 7 down then you might want to do either a cone or thermocouple offset. How many thermocouples does the kiln have and have you put cone packs in the bottom, middle and top with the same results?

It's a bit of trial and error fine tuning a kiln firing, there isn't a one size fits all kilns schedule for what works best for all glazes. For this glaze I would be aiming for a cool cone 6, so approx cone 5 1/2 - 6. If your current schedule is going at 108 to 2230 plus you are doing a 10 minute soak I'm guessing (and it really is just a guess) that if you leave the offsets for now and go to 2185F then the 10 minute soak then the rest of the program it would be somewhere to start. I'ld also look at why the kiln can't freefall at 9999 while dropping the 100F for the soak at 2100F and the 20 minute soak there. (I would drop the temp of the second soak to 100F below top temperature of 2185 to 2085)

@neilestrick, I haven't had problems with dropping at 9999F with the Genesis, have you come across this? 

Edited February 26, 2023 by Min
grammar

[neilestrick]

9 hours ago, Min said:

I haven't had problems with dropping at 9999F with the Genesis, have you come across this? 

Drops can be weird, and are often an issue. If you try to drop at too fast a rate, like 600F/hr, and the kiln can't actually drop that fast, then you'll get an error code. If you drop at 9999 it should, in theory, not have any problems because it's not trying to maintain at a specific rate. However, if it's dropping really fast like it does right after the peak temp, it can sometimes have trouble when it reaches its hold point because it takes a certain amount of time for the elements to get going again, and it can end up dropping too far and putting up an error code. The hotter the kiln is when it tries to hold the more difficulty it'll have. In multi zone kilns the 9999 rate can be a problem because the zones don't drop evenly in an uncontrolled drop, and then you get an error code due to the unevenness if they're too far out of whack. I see a lot of published cooling cycles, including from respected sources and books, that don't really understand these issues, and also don't take into account all the different factors that affect cooling like load density, kiln size, and kiln power. In general, the kiln will always be happiest if it's controlling the cooling rather than using the 9999 rate. The key is to find a rate that's fast but not so fast that it can't keep up, that makes the elements work a little bit but is still quick enough to achieve the desired results. In a short drop that's easy enough, but in a long drop you have to be aware that the rate of natural drop decreases as the kiln gets cooler. So while it may be able to maintain a 600F/hr drop at 2230F, it can't drop that fast at 1950F. In most kilns a rate of 300-400F/hr seems to be a good balance for high temp drops.

[Simon Heath]

Thank you both.

It's funny, I'm struggling a bit with this programmable kiln, because of its consistency.  My old kiln, there was almost a full cone difference between the bottom and top shelves.  Some of my glazes like it hotter than others.  I had learned to load the kiln based on results through years of trial and error.  But it was small (21X18) and manual, so now I'm trying to learn my new kiln.  It's a Cone Art and has a coil on the bottom, so my cone packs are almost identical top and bottom (haven't tested middle, there are three thermocouples).  Ideally, I'd be able to get a full kiln load of a particular glaze and fire it to its preference, but the new kiln is much bigger (23x27) and takes me close to two months to fill, depending on life!  But this is all part of the learning process, and your input is so valuable!

As for the drop, yes, I got the error on 9999, talked to a kiln tech and he suggested 500/hr.  I went overly conservative and tried 300, with no error, so maybe I will try to bump it up a bit to 400/hr.

Will see if there's any improvement and check back to post.

S.

 

Edited February 27, 2023 by Simon Heath