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MartinB

Colour Response Of Base Glazes

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Hi there,

 

So I'm fairly sure the answer to this post is 'Boron' but wanted to check with some more experienced chemists, this is all pretty new to me.

 

I did some more test tiles over the weekend and by and large they came out great. The only slight disappointment was the chrome and copper colour produced by what I'm hopping will be my 'standard' base glaze. I also tested Linda Bloomfields 'runny' base that is used across several of her glazes from her Colour in Glazes book which produced a much nicer colour. The tiles on the left are using Linda's base glaze, tile on the right is Digitatfires cone 10 transparent, the green is 0.1% chrome oxide and the turquoise is 0.5% copper addition. Hit cone 9

 

The digitalfire recipe is

 

Potash Feldspar 28

Quartz 27

Wollastonite 24

Kaolin 21

 
Linda's recipe I won't post but it has Soda Feldspar instead of Potash, at nearly double the amount, and some Calcium Borate Frit.
 
So I'm assuming the Soda Feldspar and CB Frit are fluxing it a lot more, thus the running, and the Boron in the frit is giving the nice colour response?
 
I also noticed some of the stains I tested in the digitalfire transparent are more muted compared to how they looked in a commercial transparent I bought, the purple which was a beautiful lavender in a commercial base but wasn't purple at all in this base.
 
So.... if I add some Boron to the digitalfire base (balancing the glaze) I should expect better colours out of it? And would Strontium be worth trying? Boron Frit's int he UK seem to be ridiculously expensive and I was hoping to avoid for my base.
 
Thanks for any pointers, been learning so much from this forum!
Martin

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I think the colour response is more to do with double the feldspar so a much more alkaline glaze (KNaO). I have always found a little frit at cone9/10 is worth every penny. Glaze cost per pot is in the low pennies for any glaze I have made, maybe 2p-8p based on 300 pots a bucket of glaze.

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Thanks for the reply Joel. That wasn't what I was expecting....

 

So you would add a frit, maybe Standard Borax frit or high alkaline frit, as a way of getting more KNaO into the glaze? or I should try adjusting the feldspar amount? I'm happy with the surface finish of the glaze so don't want to change too much.

 

I think I've been avoiding frits as most of the recipe's I've been interested in use Calcium Borate frit, and at Bath Potters thats £45 for 2.5kg.. The college transparent base from the previous thread you replied in probably looked a smidge better but worked out roughly £50 for 15kg, while the digital fire one was around £29 for 15kg.

 

Thanks (once) again for your help.

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Without you posting the recipe I am just making a guess that there is not much frit in the recipe and not too much boron. It will still be adding something to the colour but I think the double soda is probably the biggest help and I have assumed there is little boron.

 

Bath Potters are expensive, is that £45 before postage? I like CTM http://www.ctmpotterssupplies.co.uk/prod04.htm and their website says £50~ for 5kg calcium borate frit, nearly half price ^_^ . I did buy most materials in 10-25kg lots but I have never calculated 10kg of dry glaze material to be more than £20

 

I would think a high alkaline frit is a good start for getting somewhere in the middle of the two glazes. 2-5%

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Sodium actually can give some pretty excellent colour response by itself, and I agree that if you want to use frit as a way to add a particular oxide it's a good way to go if you want to create some interesting effects. The Digitalfire recipe is a nice, stable durable glass, and is a rather "sensible shoes" sort of glaze. It's utilitarian, it fits, and it does the job, but it's not particularly sexy.

When you're describing the Linda Bloomfield glaze as having double the feldspar (potassium and sodium do *mostly* the same thing) and some extra calcium in there as well, it makes me wonder about the amount of clay in the glaze. If you have a large proportion of alkaline feldspar and flux, and not enough silica and alumina, that will indeed make a "party shoes" sort of runny glaze that does all sorts of colourful things. It can also make for a soft, slightly soluble glass, which can be a concern with copper as an additive. (You're not likely to poison anyone, but if the glaze changes colours after someone leaves half a cup of tea out for a few days, customers get cranky.) Do a lemon juice test on one of your tiles just to check.

 

Another thing to look at (because I'm not that familiar with commonly used British kaolins or feldspars) is if there's any naturally occurring magnesium in any of your materials. That'll kill any Chrome-tin pinks that might be be the basis of your purple stain.

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I've never tried CTM but looking now they are substantially cheaper on a lot of things. Bath Potters is my go to place as they're just down the road so I can go collect, but then if they're twice the price it'll still work out cheaper shipping from CTM. Cheers!

 

I didn't post the recipe as I wasn't sure on the etiquette of posting something from a bought source, its a relatively new book out so didn't want to tread on any toes, but yep you're right, it has approx 15% Calcium Borate Frit, so the actual amount of Boron is dwarfed by the massive amount of Soda Feldspar.

 

Thanks Diesel Clay for the Sensible Shoes vs Party Shoes analogy :) Makes a lot of sense and a great way to visualise it. In my head the feldspars didn't have much to do with colour response and were just a general base, and when talking bright colours I think of Barium, Strontium and Boron.

 

The purple stain has thrown me a bit though, I'm pretty sure its not to do with Magnesium in the ingredients as I actually did a chrome/tin oxide test of both Linda's base and the digitalfire base and they produced similar results, apart from Linda's looking a bit nicer. Though I only tested the bought stain in digitalfire's and a commercial base. A brief search shows that purple stains are normally chrome and tin so it'll take some more investigation. I've attached a photo tagging which is which.

 

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You were also right about the Kaolin amount, Linda's only has about 5% which now sounds very low.

 

So on with more testing, introducing some frits and hopefully I'll end up with some sensibly party shoes. Will also be a good time to figure out InsightLive. Thankfully the studio I'm at has most ingredients you can use for testing so I don't have to commit to a big batch order yet.

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I think it is probably better not to share recipes from a new book but then it is hard to know. John Britt seems to share some of the recipes from his cone6 book in his youtube videos and such so I think it depends on the potter who wrote it.

 

15% is more than I was thinking but I still think the soda will be playing a big part. The hard part is all of the chemistry, firing and application come together to give you the final glaze so there is never one definitive answer really.

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Thanks for the input guys. I've been learning Insight Live this evening and have formulated 2 variations on the digital fire base.

 

One that introduces some boron (about half the amount of Linda's) while keeping the Ka2O and Na2O similar. The other that lowers the Ka2O and increases Na2O and has a very small bit of boron. Was a case of changing some of the feldspar's and using a little frit. The Si:Al ratio, Calculated Expansion and CaO values have stayed pretty much the same as the original recipe, was fun trying different ingredients and balancing everything out.

 

Will be interesting to see what the change in flux ratio does to the colour response and viscosity, I'll post results when they're done but might be a couple of weeks.

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I'm curious why you lowered potassium and raised sodium? They should do pretty damn close to the same thing, I would think

 

Thats what I originally thought but from what the others have said and some further reading higher sodium content can give a good colour response turning Copper turquoise and Chrome a strong greeny/yellow, which is whats happened with my test tiles I think.

 

Linda's base is high in Sodium and low in Potassium, and the digitalfire base is the opposite. So I'm trying to push the digitalfire base towards Linda's, but without going too far (I don't want a really runny glaze). I think the Boron still has something to do with the colours so going to try that too. The rest of the ingredients and amounts of them aren't drastically different, so its got to be the amounts and ratio of the fluxes.

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So the two new glaze bases came out the kiln this morning and they definitely improved the colours, though I foolishly didn't label them correctly so don't know which was the higher sodium and which was the higher boron.

 

The copper at 0.1 percent looks good and matches the colour of Linda's Runny. The Chrome at 0.1% didn't go as bright as Linda's, but definitely looks better than the dull previous tile and these were 100g batches so not sure how accurate my 0.1g addition was, and the Purple Stain is now working again.

 

All in all I'm happy, learnt a ton about glaze chemistry along the way :)

 

Chrome at 0.1%

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Copper at 0.1%

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Purple stain at 4%

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