Jump to content


Photo

A Question About Glaze Calculators


  • Please log in to reply
4 replies to this topic

#1 yedrow

yedrow

    Advanced Member

  • Members
  • PipPipPip
  • 405 posts

Posted 20 January 2013 - 03:52 PM

When I put Calcium Carb. and Silica in the calculator I get a corresponding increase in CaO and SiO2. But, when I remove them and put an equal amount of Wollastonite into the calculation the CaO increases but the SiO2 decreases. What am I missing here?

Joel.

#2 JBaymore

JBaymore

    Moderator

  • Moderators
  • 2,473 posts
  • LocationWilton, NH USA

Posted 20 January 2013 - 11:30 PM

Your description is not all that clear....... but..... as a guess here.......

.......likely it is related to the concept of the Unity formula. The total of all of the fluxes always are adjusted to equal one in a Unified Seger Molecular Formula.


best,

.......................john
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#3 yedrow

yedrow

    Advanced Member

  • Members
  • PipPipPip
  • 405 posts

Posted 21 January 2013 - 05:27 AM

Ack, second time typing all of this...

I see your point, let me see if I can clarify.

The recipe is as follows:

Custer 20.0
Whiting 20.0
Wollastonite 20.0
EPK 20.0
Silica 20.0


The chemistry reads for CaO and SiO2 as follows:

Unity Analysis

CaO 0.92 23.72
SiO2 2.19 60.38

If I add 10 to the wollastonite the chemistry changes thus:

CaO 0.93 26.21
SiO2 1.98 59.50

Then, if I remove 5 from whiting and 5 from silica, the chemistry reads:

CaO 0.93 25.37
SiO2 2.01 59.10

Granting I'm not a chemist, and I've only begun using this software recently, the errors are likely mine. But, it seems to me that since the wollastonite has no LOI and it chemistry is, 51.7% SiO2, it seems that with the last changes the SiO2 should be about the same, less any changes from the LOI of the CaO.

Most of the time the software does what I expect it to. Outside of this oddity, when it doesn't do what I'm expecting, I'm expecting the wrong thing. Since I'm in the learning process such an apparent discontinuity is tripping me up since it suggests I could be all wrong about what I think is happening in other places.

Joel.

#4 JBaymore

JBaymore

    Moderator

  • Moderators
  • 2,473 posts
  • LocationWilton, NH USA

Posted 21 January 2013 - 12:21 PM

Joel,

Note that when you increase the Wollastonite content in the way that you did..... what you are doing is somewhat "counter intuitive" until you grasp the "big picture".

Yes, you are adding a matterial that has roughly 1/2 mol silica and 1/2 mol calcium for every mol of wollastonite. However as a percentage of the total batch recipe, you have increased the overall share that wollastonite takes up as a percent of the total... and DECREASED the portion that silica/flint takes up. By upping the wollastonite to 30 parts... you have increased the percentage of the total raw materials that IT takes up to 27.27% (up from 20%).... while decreasing the percentage of silica to 18.18% and whiting to 18.18% (and all other original 20% components).

For every 1% of wollastonite you increase as a pecentage of the recipe, you are increasing the SILICA content introduced by it by only 1/2 of that (wollastonite is only roughly 1/2 silica by mols). But at the same time as you decrease the flint / silcica content by 1%, almost ALL of that decrese is actually concentrated as silica in the fired formula. So "one half step forward, one step back".

Then you also have to factor in the decrease in the silica (and other oxides) supplied in the formula coming from the decrease in the percentages of both Custer and EPK from 20% to 18.12%. Custer supplies a tad of CaO.... at about .03 mols.... but supplies a LOT of silica at about 7.2. EPK supplies a trace of CaO, but about 2 of silica.

Also note the K2O and Na2O and Al2O3 drops that are occuring for the same exact reasons. The PERCENTAGE drops on Custer and EPK content are accounting for that set of changes.

The calcium oxide goes up....... but not by all that much. This is again mainly because of the decrease in the percentage of calcium oxide which is being supplied by the whiting. Whiting is supplying almost only CaO, and is the main source other than the wollastonite. Wollastonite again is supplying only about 1/2 of its percentage content as CaO in the fired form. Again........ "one half step forward, one step back".

Now there is another factor involved here......... and I'll use the whiting to serve as the example here.

Whiting is calcium carbonate. One unit of calcium carbonate supplies one unit of calcium oxide in the fired form. HOWEVER...... when you WEIGH OUT an amount of calcium carbonate.... you are weighing out a raw material composed of one atom of calcium, one atom of carbon, and three atoms of oxygen. All of those atoms have mass... and in a gravity field (like Earth's) .... have weight. So we have to take into account what is LEFT in the fired form when you take whiting and heat it up in a kiln. We know that the ceramic active chemistry in the glass from whiting is CaO....... so from that we also know that CO2 is given off from the whiting to result in CaO. This CO2 goes off as a gas with the kiln effluent. This accounts for the factor known as the LOI (Loss on Ignition).

That change factors in here a bit also. Relationships of molecular weights is a HUGE portion of the understanding of glaze chemistry when it comes to batch to molecular and molecular to batch conversions.

That's the "bulk" of this. Does that help you understand this?

Remember to hit "retotal recipe" when you make such weight changes as you are working and keep it at a percentage recipe rather than a batch recipe... it helps you to see these things coming at you.

best,

.....................john
John Baymore
Immediate Past President; Potters Council
Professor of Ceramics; New Hampshire Insitute of Art

http://www.JohnBaymore.com

#5 yedrow

yedrow

    Advanced Member

  • Members
  • PipPipPip
  • 405 posts

Posted 21 January 2013 - 11:14 PM

Thanks John. That was a great help. I was hoping it was just a math error on my part, and it was. I hate math, grrrr. You offered perspective from an expert, and that makes a huge difference.

But at the same time as you decrease the flint / silcica content by 1%, almost ALL of that decrese is actually concentrated as silica in the fired formula. So "one half step forward, one step back".



Is this because free silica doesn't enter the melt in the same way that molecularly bound silica does, as in: flint verses the compounds, clay and wollastonite?

This CO2 goes off as a gas with the kiln effluent. This accounts for the factor known as the LOI (Loss on Ignition).


It seems that the contributing effect of LOI is greater than I was thinking. {I just did the math right this time} I got it to work out pretty close with 30 Wollastonite, 15 SiO2, 9.4 CaCO. Thanks.

Remember to hit "retotal recipe" when you make such weight changes as you are working and keep it at a percentage recipe rather than a batch recipe... it helps you to see these things coming at you.


I didn't think about it that way. I've been retotaling at the end, thanks again. Now things are making much better sense.


Joel.




0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users