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GlazeChem Limit Formulas
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rjw  3/25/97

  GlazeChem versions 1.2 and later include a Limit Formula
database. This file gives a brief overview of the new features
related to limit formulas. More information is available in the
online help.

2/24/01: several sets of limit formulas are included with
GlazeChem, in different limit formula databases. By default the
Cushing limit formulas are loaded.


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Limit Formulas
--------------
  Limit formulas are general guidelines for the chemical
composition of different categories of glazes. For example,
in "Clay and Glazes for the Potter", Daniel Rhodes gives a
limit formula for cone 8-12 Stoneware or Porcelain glazes
as follows:
      KNaO   0.2 - 0.4
      CaO    0.4 - 0.7
      MgO    0.0 - 0.35
      ZnO    0.0 - 0.3
      BaO    0.0 - 0.3
      Al2O3  0.3 - 0.5
      B2O3   0.1 - 0.3
      SiO2   3.0 - 5.0

  The values given for each oxide are the unity formula
amount minima and maxima usually expected for a c/8-12
stoneware glaze. Limit formulas can help you to adjust
glazes for different effects, different firing temperatures,
greater durability, etc. Limit formulas can be used as
starting points in constructing your own glazes from scratch.

  In GlazeChem, you can set limits on either the unity formula
amount of an oxide, or on the weight percentage of an oxide.
There is presently no way to set limits on unity amount
percentages; these must be converted to unity amounts.


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Oxide Combinations
------------------
  Traditionally, limit formulas contain limits on the amounts
of the different oxides in a glaze - Na2O, K2O, CaO, MgO,
Al2O3, SiO2, etc. Sometimes you may also find limits for
pairs of oxides (KNaO in the example above), or for the
relative percentages of fluxes, alumina, and glass-formers.

  GlazeChem covers all these possibilities and more through
the use of "oxide combinations." An oxide combination can be
a single oxide (i.e. "Na2O"), a sum of any number of oxides
(i.e. "K2O+Na2O" or "CaO+SrO+BaO"), a difference between two
oxides or sums of oxides (i.e. "K2O-Na2O"), or the ratio of
two oxides or sums of oxides (i.e. "SiO2/Al2O3" or
"SiO2/(Al2O3+B2O3)").

  Most users will not use oxide combinations more complex than
single oxides or simple sums or ratios. The more complex forms
are provided so that interested users can create and explore
nontraditional limit formulas.


--------------------------
The Limit Formula Database
--------------------------
  The limit formula database contains a list of limit formulas,
and a list  of the oxides and oxide combinations from which the
limit formulas are constructed. GlazeChem opens a limit formula
database automatically when it starts (it opens the one you had
open the last time you used GlazeChem). There can be only one
limit formula database open at a time; if you wish, you can close
the current one and open another, or start a new one from scratch.
Limit formula databases are stored in files with the extension .LIM.

  The limit formula database is displayed in the Limit Formula List
view, in the form of a scrollable list of limit formulas. Individual
limit formulas are viewed in more detail, and edited, in the Limit
Formula Edit view. These views are similar in form and function to
the list and edit views for other GlazeChem databases.

 The oxide combination list is edited in the Oxide Combinations
dialog box. This dialog can be opened by choosing Utilities |
Oxide Combinations from the Limit Formula List or Edit view.
You need to set up any new oxide combinations here before you can
use them in limit formulas. Please remember to keep oxide
combination names short, 8 characters or less, so that they fit
in all the various places where they must be displayed. (Hopefully
I'll be able to up the 8 character limit to 12 or so when the
character graphics in the Glaze Edit view are replaced by real
graphics.)

  I won't explain the detailed workings of the Limit Formula Edit
view or the Oxide Combinations dialog here; if you've been using
GlazeChem a while, you will probably be able to figure them out.
They are explained in detail in the online help. Remember, if you
need help with any view or dialog in GlazeChem, press F1 or click
on the nearest "?" button to get information from the online help.


-------------------------
Limit Formulas and Glazes
-------------------------
  With regard to glazes, the most important new feature is the
Limit Formula page of the Glaze Edit view. You can see it by
clicking the Limit Formula button at the top of this view. On
this page, there is a list of limit formulas at the left, and a
box at the right where the limit formula and glaze are compared.

  If the glaze analysis is known (all ingredients are in the
material list), the limit formula list tells you which limit
formulas this glaze falls into. These glazes have a "*" in front
of their names. One limit formula may also have a "." in front.
This is the "associated" limit formula for the glaze; it is the
limit formula to which this glaze is compared in the Glaze Report.
You can set the associated limit formula by clicking the Set
button.

  One limit formula in the list may be selected. The glaze is
compared to this limit formula in the box at right. A typical
comparison looks like this:

        0.0                 0.5                 1.0
    KNaO :.|-;---;---;---;---|...:...o...:...:...: 0.71
     CaO :.|-;---;---;---;--*;---;---;---|...:...: 0.47
     MgO |---;-|.o...:...:...:...:...:...:...:...: 0.19
     BaO *---;-|.:...:...:...:...:...:...:...:...: 0.00
     ZnO |---*-|.:...:...:...:...:...:...:...:...: 0.10
    Li2O *---;---;---;---|...:...:...:...:...:...: 0.00
     SrO *---;-|.:...:...:...:...:...:...:...:...: 0.00

        0.0                 1.0                 2.0
   Al2O3 :...|---;-|o:...:...:...:...:...:...:...: 0.56
    B2O3 *---;---;-|.:...:...:...:...:...:...:...: 0.00

        0.0                 5.0                10.0
    SiO2 :...:...*---;---;---;---|...:...:...:...: 2.03

  The oxide combinations down the left side are those contained
in the selected limit formula. For each oxide combination, a
graph is plotted from the plot minimum to the plot maximum for
that combination (these are specified in the Oxide Combinations
dialog box). On the graph, GlazeChem plots the allowable range
for the combination (as given by the limit formula) as dashes
between vertical bars. On top of this, GlazeChem shows the
glaze's calculated value for the oxide combination. It is shown
either as a "*" (if it falls within the limits) or as a "o" (if
it falls outside the limits). The value is also shown at the
right of the graph.

  In a future version you will have the option of showing this
graph as graphics rather than text characters. Text does have
its advantages, though - you can easily copy and paste the chart
to another document or to email.

2/24/01: The graphic display mentioned above has been implemented
in version 2.1.

  Another future version will make it easier to adjust the glaze
to the limit formula. At present, you need to jump back and forth
from the Recipe page to the Limit Formula page, and the program
doesn't help you decide which material to increase or decrease.

-----------------------------------
Creating Limit Formulas from Glazes
-----------------------------------
  New limit formulas can be created from a set of glazes by
analyzing the glazes for the minimum and maximum values of oxides
and oxide combinations. To do this, use the Make Limit Formula
dialog box, opened by choosing Utilities | Make Limit Formula in
the Glaze List view. See the online help for more information
about how to use this dialog.

  Note that if you select the Statistics option, the new limit
formula's notes will contain statistics about the glaze analyses,
including histograms.


