Effect of Magnesium on Glaze Melt at Cone 6

[BobMagnuson]

I decided to run a couple experiments to test some of the recipes discussed here earlier  Both were fired to cone 7.  I work at a community studio (Duluth Art Institute) and cone 7 was the next firing...I didn't want to wait for the next cone 6. Close enough.

First, I made up a batch of Min's "Base Test." (See Oct 1.) Then I took the same recipe and left out the talc but replaced the amount of silica that the talc would have provided.  In other words, I just took out the MgO.  My test tiles were Continental Clay "B Clay", bisqued to cone 08 and scribbled with a black underglaze pencil.  The difference was not dramatic, but I would say the "Base", with MgO, is more "fluxed" than the modification, without MgO. (Notice how the underglaze ran more when MgO was present.)

Then, I made up a batch of Bill Kielb's "Marcia's Matte", (See Oct 2) and added 10 and 20% of SiO2.  Once again, the results were not dramatic, but the added silica did indeed make the glaze glossier, though still somewhat matte. Notice how the scribbling on the original is a bit less black than the other two.

My conclusion is...yes, MgO can act as a flux in midrange glazes.  

I have to admit that I haven't worked with midrange glazes for quite a while. I've been working at cone 8 to 10 for the past couple of decades.  And I rarely use borates for high fire.  It just seems like cheating!

My original thinking was this: if it's so easy to make a high fire magnesium matte, how can MgO be a flux at a lower temperature?  Maybe borates are the key.  Maybe.  Where to go from here?  Surely there's no simple answer.

[Bill Kielb]

19 hours ago, BobMagnuson said:

but the added silica did indeed make the glaze glossier, though still somewhat matte. Notice how the scribbling on the original is a bit less black than the other two.

Marcia’s matte was an example of a Mgo glaze that will go matte to gloss with nothing other than the addition of silica. Actually the surface change should be quite dramatic and your pictures do show a significant difference in clarity and surface gloss. Actually, this glaze will go very glossy with more silica. It was an example that matte is not necessarily the same as an opacifier which occurred earlier in the thread. Interestingly your pictures seem to illustrate that point quite well.

[Min]

On 11/1/2023 at 8:10 PM, BobMagnuson said:

I made up a batch of Bill Kielb's "Marcia's Matte", (See Oct 2) and added 10 and 20% of SiO2.

Did you do this with and without the MgO or was it just to demo the SiO:Al2O3 ratio effect on gloss levels?

[BobMagnuson]

I did the Marcia's Matte demo without any recipe modifications.   If MgO is just a matting agent, I thought,  how can adding silica reverse that effect?  I don't know, but it did.  One possible explanation is, since the effect was not dramatic, the extra silica simply reduced the percentage of talc, and hence MgO,  in the recipe.

[Bill Kielb]

15 minutes ago, BobMagnuson said:

since the effect was not dramatic,

I’m curious what the not dramatic reference means? I have seen these recipes do this so many times just to illustrate the effect that in general they follow the trend of the Stull map. Typically the recipes tested will go dry matte to full gloss once above 7:1 Si:Al to let’s say to 9:1. From a surface texture perspective, and gloss meter reading I believe that would be dramatic.

Edited November 4, 2023 by Bill Kielb

[Min]

1 hour ago, BobMagnuson said:

If MgO is just a matting agent, I thought,  how can adding silica reverse that effect?  I don't know, but it did.

MgO is in the role of a flux, glaze in question is a high alumina matte, adding silica doesn't negate the fluxing of the MgO it moves the SiO:Al2O3 ratio enough to move from a matte to a gloss due to the increase of SiO. 

edit: was the kiln slow cooled?

Edited November 4, 2023 by Min

[Bill Kielb]

7 minutes ago, Min said:

MgO is in the role of a flux, glaze in question is a high alumina matte, adding silica doesn't negate the fluxing of the MgO

I agree, this was offered in support of Neils observation that matte and opacifier have very different meanings and I agreed with his observations as being consistent with my experience. So not intended to be a part of the Mgo flux debate other than from that perspective. Not dramatic is what confuses me with respect to matte / gloss. For this glaze and many others dialing in the amount of matte is simple by addition of silica (no special cooling required) which can be a handy tool to use in glaze composition.

[BobMagnuson]

The tile on the left is Marcia's Matte.  On the right is the same glaze with 20% silica added.  It's hard to see in a photo, but the silica made the glaze less matte, but not glossy.

I think the matte in Marcia's Matte is created from both excess Al2O3 and MgO.  Adding silica changes the SiO2:Al2O3 ratio, but doesn't necessarily affect what MgO is doing.  

I will agree that I have used the definitions of opacifier and matting agent too loosely, but they are often the same thing.  Creating a matte surface automatically creates some opacity.  The reverse, however is not true.

From https://digitalfire.com/oxide/mgo:  "At lower temperatures the matting mechanism of MgO is that it simply stiffens and opacifies the glaze due to its refractory nature."

[Min]

Adding 20 SiO2 only brings the SiO2:Al2O3 ratio up to 6.1:1 (using Insight data) so I'm not surprised it isn't very glossy but it does look like it's moved it a bit towards a gloss. There is going to be a limit to the amount of SiO2 the fluxes can incorporate before running into the issues of undissolved silica itself causing matting.

Re the quote from Hansen above, is he talking low fire when he says "At lower temperatures the matting mechanism of MgO is that it simply stiffens and opacifies the glaze due to its refractory nature."?

[Bill Kielb]

5 hours ago, BobMagnuson said:

The tile on the left is Marcia's Matte.  On the right is the same glaze with 20% silica added.  It's hard to see in a photo, but the silica made the glaze less matte, but not glossy.

Much better photo! I’ve done this lots of times to illustrate the trend in Stull so I know this glaze will go full gloss with more silica. Not that’s how one would like to achieve the ideal gloss glaze. This is a common Stull observation and is not limited to magnesium mattes. Sue McLeod does her demo with a calcium matte I believe. The surface generally can go dramatically from dry matte to a gloss and becomes a technique to dial in a desired gloss level. I do agree the diffraction caused by the matte creates the appearance of opacity, but to Neil’s point, there are many glazes in my experience that are glossy and clear yet contain considerable Mgo so opacifier seems maybe not the best term.

Sues glaze https://suemcleodceramics.com/how-to-turn-a-matte-glaze-glossy-with-one-ingredient/

Edited November 5, 2023 by Bill Kielb