Min

This thread is jumping off from this one where the subject of the efficacy of magnesium as a flux in a cone 6 glaze came up and the prospect that the magnesium alumino-silicate eutectic is unlikely to form at cone 6. It's probably best to read that thread for background before this one. To test whether or not magnesium acts as a flux at cone 6 I'm going to run a test with the glaze below. Purpose of the test is to see what effect the MgO has on the melt. I know the first version of it does melt well at cone 6 and results in a clear gloss glaze. For the second version I have removed the talc (that supplied nearly all the magnesium). I have rebalanced the silica and alumina but left everything else the same. Testing if the removal of the talc effects the melt fluidity of the glaze. In theory the first glaze should have more fluidity if the MgO is acting as a flux. There is 0.1 MgO left but this is as low as I can get it without messing around with the other materials. It's a bit of a wonky recipe insofar as materials used and totals as it was part of an experiment I was doing blending different recipes. @BobMagnuson, does this look like a suitable recipe candidate for this test? Also, if anyone has the eutectics for a boron calcium magnesium silica alumina melt that would be terrific! Or even one with boron magnesium alumina silica. I found some for boron + magnesium here. The last column is in degrees C.

Have a look at this article, it goes into how MgO works at mid range acting as a flux at around ^6 compared to lowfire where it acts more as a refractory agent causing the glaze to opacify. Yes, there is a limit to how much of it will be taken into the melt. I'll start a new thread on how MgO effects glaze melt.

Adding one more suggestion, take some of your claybody and make it into a very thick slip.Add a drop or two of Darvan to about a cup of the slip then brush it through a 60 mesh sieve. Once the pots are leatherhard brush or dip a thin coating of slip over the rims and base/foot. I would do this after burnishing the grog back into the clay. Since the slip is the same as the body there won’t be glaze fit or colour issues. The grog free slip will give you a much smoother finish.

Super interesting for sure! My clear has .25 magnesium (from frit) in the unity formula. I’ll try and remove it to get rid of the magnesium and use a magnesium free frit(s) to keep frit percentage the same (given Frits in general melt easier than most raw materials) and see what happens. If you have a suggestion for any glaze test comparisons let me know and I’ll run those too. I’m firing (^6) next week. another edit: I'm going with a MgO matte (with slow cooling) that used talc to supply MgO instead. I have stumbled across an article from NASA where they used calcium, magnesium (plus silica & alumina) to make a glass, I seem to recall the melting point wasn’t exceptionally high, I’ll try and find the article tomorrow. I think there was a phase diagram in the article. edit: @Marie Lu, we have gone off topic a bit here. Debating over the effect of magnesium in regards to fluxing at^6, I do think it acts as an auxiliary flux but open to discussion and experimentation.

I don't use Stroke & Coat so I went ahead and emailed Mayco, their response below. How well the underglaze will show through is something you will have to test.

Interesting comments about magnesium. So in your view is what Linda Bloomfield says about magnesium being an active flux above 1170C / 2138F incorrect? Also, do you have the melting point for a CaO + MgO + SiO2 Al2O3 eutectic?

Have a great trip Mark! When you get back it might be worth trying some titanium dioxide + iron or iron silicate and see if that works. Either way it could rule in or rule out one of the variables.

What qualities are you looking for in a white glaze?

There are different methods of creating a matte glaze. Some matte glazes employ more than one mechanism to do so. The mechanism and oxides used will create different types of matte, some will feel butter soft (like a good magnesium matte), others will feel dry (like one created with barium), some cutlery mark easily (often these are high alumina mattes that don't use a secondary mechanism or oxide to create the matte) and so forth. Oxides that when used in higher amounts can cause matting are magnesium, barium, strontium, titanium, zinc, calcium and alumina. There are also underfired glazes that will be matte but this is simply because they haven't been fired hot enough to melt properly. Slow cooling a glaze is one method of achieving a matte glaze if the glaze is high in one or more of the matting agents. Particular oxide(s) combine with silica and precipitate out to form a microcrystalline glaze. These micro crystals will scatter the refracted light causing a matte surface. In effect what is happening is the glaze is devitrifying as it cools. Since these matting oxide(s) + silica micro crystals have a different composition from the remaining glaze I would argue the calculated R2O:RO flux ratio is no longer valid. There are exceptions to the slow cooling method of achieving a matte glaze, barium carb for example will cause a matte glaze regardless of the cooling schedule. Adding excess alumina oxide will also cause a matte as an excess of it will simply not be taken into the glaze melt.

@BobMagnuson, how about if the magnesium is supplied by a frit, do you think it would be more likely to form a magnesium alumina silicate eutectic at ^6? Lone wolves ideas are more than appreciated chiming in on this, I've always thought there are more variables than just the R2O:RO ratio alone. What about when a slow cooled matte glaze has some of the calcium/strontium/magnesium etc precipitate out of the glassy matrix, does the calculated flux ratio still apply, I don't think it does, at least not in the matting oxides entirety at least.

So at 1080C (approx cone 03) the clay will be less dense and mature than if fired to the upper temp of 1180C which is approx cone 5. The more mature a clay is the stonger it is. For what you are making it probably isn't critical to fire to 1180C but if you ever decide to make functional ware it would be a good idea to do fire to the clays maturity temp of 1180C. Another point is the colour of the clay will in most likelihood change as you fire hotter. Below is an example of a local earthenware clay I can get fired to different temperatures.

I have seen quite big differences in the maturity cone for earthenware bodies in the UK. For the clay you are using what is the top cone (or firing temperature if a cone isn’t posted) for the clay?

Hi Marie and welcome to the forum! My experience with the 5% soda ash test when testing the black glaze Licorice from the book Mastering Cones 6 Glazes by Hesselberth and Roy led to a degradation of the glaze gloss level. I then went on to leave a test piece in the dishwasher for several months. Real world dishwasher tested did not show any degradation. I do think the soda ash test is too harsh a measure. Note it is within a hair from the "ideal" flux ratio. I did contact Roy about this and he didn't recall doing the soda ash test with this glaze. If your glaze can take it increasing the silica and alumina in a formula will improve durability regardless of flux ratios. Regarding the "ideal" flux ratio of R2O: RO of 0.30:0.70, have a read of this snippet from Matt Katz's 2016 article Glossed Over: Durable Glazes from NCECA 2016 note the test for this article with a 0.1:0.9 ratio was "surprisingly robust" and the need for further research regarding the effects of silica:alumina ratio and the use of colourants. I don't think we are at an absolute conclusion yet in regards to flux ratios. They do not stand alone insofar as durability measures.

I'ld run it through the finest sieve you have then let the undissolved residue of soda ash dry in the sieve then dump it out and pour very hot water over the residue and add it back to the glaze slurry. (If you try and scrape the undissolved residue out while it's still wet it's harder to get all of it out)

@SunsetBay, my preference is to have at very most EPK in a recipe at 20%. If you calcine some of the EPK it will reduce the shrinking (and gelling in the bucket) of both glazes. EPK has a LOI (loss on ignition) of 13.20%. What this means is you need to reduce the amount of calcined EPK in the recipe by 13.20% (by weight) from the original amount. If you use 20% as the maximum of raw EPK then make up the balance with calcined EPK the V.Slate Blue would be 20 EPK plus 8.68 calcined EPK. Spearmint would become 20 EPK plus 6.94 grams of calcined EPK. Measure the specific gravity of the glaze when you test it and also take note of number of dips and how many seconds you dip for. I would suggest you do some test tiles to check this and see which result you like the best. If you find it still crawling then decrease the amount of EPK further still and replace it with calcined EPK using the same math. Subtract 13.20% of the weight from the calcined EPK. To calcine EPK just put some in a bisqued bowl and run it through a bisque firing. It's handy to have a supply of it ready to use. I would suggest dropping the bentonite altogether, don't need it in either recipe.

Are you using the rutile for its visual texture only or for its colour also? I recalculated my recipes using titanium dioxide in place of rutile as I use it for visual texture. (Sorry about going off topic here Pres)

What changed? New batch?

+1 for what Bill said. There are a couple versions of the Mc6G Licorice glaze since the original had a fair bit of EPK in it which caused crawling for some people when it was applied heavily. One of the altered versions used calcined epk for part of the total epk and the other used a combo of epk plus some ball clay. How much clay is in the Spearmint and Varigated Slate Blue? Recipe versions that you are using?

I'ld do it again to your usual cone once you have your kiln sorted out.

In addition to what Tom said the covering glaze would be somewhat blended with the crazed base glaze when they are fired. On a side note, I have some pots I use daily at home that are vitrified but partly unglazed on the outside. When testing this clay I wanted to make sure it didn't stain from everyday use where it is unglazed. I soaked the fired samples in beetroot water, strong tea and coffee and sumi ink. Nothing stained it and years later the pots still look pristine. Takeaway from this (for the Iron Wire and Golden Thread) would be to use a body that doesn't totally vitrify if you use a stain that isn't fired on like a china paint.

@Christy Ann, while you are doing your testing I would suggest doing some absorption tests with your claybodies also. Manufacturers post absorption figures but it's best to do your own testing since those posted figures can vary from actual results using our own kilns and firing practices. Info on how to do a calculated absorption test on this link about 2/3 the way down. Given the posted absorption figures are fairly high for the clays you are using and typically ^6 and ^10 work isn't glazed on the bottoms (like lowfire or earthenware usually is), this is relevant. As has already been said the absorption figures for functional ware at mid and high fire should be below 1 1/2 - 2%. While you can get a well fitting non crazing glaze for pots fresh out the kiln they should still be stress tested for delayed crazing also. When functional pots (with higher absorption) is used it soaks up moisture from use and from doing the washing up. This moisture enters a porous claybody causing it to expand ever so slightly. (especially when heated as in a microwave) When the claybody expands two things happen: one is the clay can become very hot, as in too hot to pick up a mug by the handle, and secondly the glaze will more than likely craze. This is called delayed crazing. Delayed crazing can also happen over time even without using the pot if the absorption is quite high by atmospheric moisture. edit: to stress test for delayed crazing put some samples (fired in different areas of the kiln) into a 300F oven. Leave them there for 20 minutes or so then plunge them into ice water. On a clear glaze if there is crazing it should show up, if unsure cover it with a felt pen then wipe it off. (or use calligraphy ink if you have it)

I love how curious you are about so many different techniques and styles! Stain the initial primary crazing then leave the pot for the secondary craze lines to develop unstained.

I've found that using a soft clay helps prevent the clay from pleating or buckling when collaring in. I also leave the wall a bit thicker than the lower part of the form where I start to collar it in, collar in a bit then throw that then collar in again and repeat until it is a narrow (or enclosed) as needed. Watching videos is great as long as the person doing the demo knows what they are doing. I had a quick look at Florian Gadsby's one on throwing a simple bud vase, at around the 5 minute mark he collars the clay in.

I believe the image in the first post is a work by Yō Akiyama and is from his work using stoneware, heat and peeling off layers of clay to achieve the cracks. If you search his name you can find many examples of his work. From this Wikipedia link: "Akiyama developed and fine-tuned the latter technique in the 1980s while experimenting with the feasibility of peeling the outer skin off a ball of clay, in the same way that one might peel a fruit. To achieve this, he heated a ball of clay with a gas burner, creating a shape with a soft center and a hard, outer shell. Since that first experiment, Akiyama has refined his technique and manipulated this template to create a multitude of cracks and chasms on the surface of his pieces, in some cases completely inverting the shape."

Veegum T does make clay lovely and plastic doesn’t it. Now if only it wasn’t so expensive!