Molecular Unity Formula or Seger Formula
This is a method of deriving the oxide amounts from a glaze recipe. It uses the relative weights of all the atoms that are in the fired glaze and organises how many there are of each sort relative to one another. It is important to note that the molecular formula is an abstraction that is based on what actually happens. So in a molecular formula it is common to talk about fractions of molecules, when in reality that isn't the case, but the important information isn't about the absolute numbers of molecules in a glaze (which would be countless billions) but rather the relative numbers of each.
To start the process, first list the ingredients:
(for our example we will be using the Leach Limestone glaze)
| Potash Feldspar | 40 |
| China Clay | 10 |
| Whiting | 20 |
| Silica | 30 |
Next we find the list of oxides that makes up each ingredient and what it's molecular weight is. Molecular weight isn't an actual physical weight, instead all the atoms are ranked from lightest (hydrogen) to the heaviest (ununbium), hydrogen was given the molecular weight of 1 and the weights of all the other elements are expressed in terms of their relationship to the weight of the hydrogen atom. For example the atom oxygen is sixteen times as heavy as hydrogen so its atomic weight is 16. The molecular weight of a material is just the sum of all the atomic weights that make up the material's formula. So for a material like china clay with the formula; Al2O3.2SiO2.2H2O the weights are added up like this;
Al = 27 X 2 = 54
O = 16 X 3 = 48
plus
Si = 28.1 X 2 = 56.2
O = 16 X 4 = 64
plus
H = 1 X 4 = 4
O = 16 X 2 = 32
Total = 258.2
Now we layout the glaze recipe using the above information:
| MATERIAL | FORMULA | AMOUNT | MOLECULAR WEIGHT | CALCULATED MOLECULAR WEIGHT | ||
| Potash Feldspar | K2O.Al2O3.6SiO2 | 40 | ÷ | 556.8 | = | 0.0718 |
| China Clay | Al2O3.2SiO2.2H2O | 10 | ÷ | 258.2 | = | 0.0387 |
| Whiting | CaCO3 | 20 | ÷ | 100.1 | = | 0.2 |
| Silica | SiO2 | 30 | ÷ | 60.1 | = | 0.499 |
Next we multiply the individual oxides in each material by the calculated molecular weight (C.M.W.) of the whole material. At this stage we can discard any molecules that will be vaporised by the kiln when heating up, these molecules are chiefly water (H2O) and carbonates (CO3):
| MATERIAL | C.M.W. | K2O | CaO | Al2O3 | SiO2 |
| Potash Feldspar | 0.0718 | 0.0718 | 0.0718 | 6X.0718=.4308 | |
| China Clay | 0.0387 | 0.0387 | 2X.0387=.0774 | ||
| Whiting | 0.2 | 0.2 | |||
| Silica | 0.499 | 0.499 | |||
| TOTAL | 0.0718 | 0.2 | 0.1105 | 1.0072 |
Now we list these numbers under the various oxide groupings. From this point on we have discarded the idea of materials and are solely working with various quantities of oxides. We group the various oxides under headings as already explained, we also group them using a system that refers to the amount of oxygen present in the molecule. So all molecules that have only one oxygen go under the R2O group (the R stands for radical and can be substituted for any atom), these are also usually fluxes or opacifiers. Molecules with three oxygens for every two other atoms are under the R2O3 group and thought of as stabilisers. The last group is for molecules with two oxygens for every other atom, RO2 and are the glass forming oxides.
| FLUXES (R2O) | STABILISERS (R2O3) | GLASS FORMERS (RO2) | |||
| K2O | .0718 | Al2O3 | .1105 | SiO2 | 1.0072 |
| CaO | .2 | ||||
| TOTAL | .2718 | ||||
This is now a molecular formula. However to be more useful and to comply with the accepted ceramic industry norm we perform an additional calculation that makes the first column equal one, hence the term Unity Molecular Formula. We do this by dividing each amount by the total from the flux column (R2O):
| FLUXES (R2O) | STABILISERS (R2O3) | GLASS FORMERS (RO2) | |||
| K2O | .264 | Al2O3 | .406 | SiO2 | 3.7 |
| CaO | .736 | ||||
| TOTAL | 1 | ||||
From this we can now derive the important ratios:
Silica:Fluxes Ratio = 3.7:1
This is the correct amount of silica for a cone 8 glaze when using limit charts.
Silica:Alumina Ratio = 3.7:0.406 or to simplify we divide each side of the ratio by the alumina figure
E.g. 3.7÷0.406=9.1, 0.406÷0.406=1, so the ratio is 9.1:1
This ratio would put the glaze in the glossy end of the spectrum.
Flux Ratio = Alkali .264:Akaline Earth .736
Indicates the glaze has the correct balance for a high temperature glaze.
The table below lists all the common elements used and the molecular weights for all the common materials used in glazes.
| NAME | FORMULA | Molecular Weight |
| Potash Feldspar | K2O.Al2O3.6SiO2 | 556.8 |
| Soda Feldspar | Na2O.Al2O3.6SiO2 | 524.6 |
| Nepheline Syenite | K2O.3Na2O.4Al2O3.8SiO2 | 1169 |
| Petalite | Li2O.Al2O3.8SiO2 | 612.6 |
| China Clay | Al2O3.2SiO2.2H2O | 258.2 |
| Wollastonite | CaSiO2 | 116.2 |
| Talc | 3MgO.4SiO2.H2O | 379.3 |
| Lead Bisilicate | PbO.2SiO2 | 343.4 |
| Colmanite | 2CaO.3Ba2O3.5H2O | 411 |
| Lithium Carbonate | Li2CO3 | 73.8 |
| Whiting | CaCO3 | 100.1 |
| Dolomite | CaCO3.MgCO3 | 184.4 |
| Magnesium Carbonate | MgCO3 | 84.3 |
| Barium Carbonate | BaCO3 | 197.3 |
| Zinc Oxide | ZnO | 81.4 |
| Silica | SiO2 | 60.1 |
We have a tool called limit charts that are very useful for analysing a glaze once it's in the Unity Molecular Formula. These have been worked out for various temperatures and list the optimum amount for any oxide. It is important to note that the effects of combining various oxides can produce unexpected results even when using these charts . Also they are designed only to provide a shiny stable glaze, if you want a satin or matte finish then you will need to depart from the limit charts to achieve this. These limit charts were drawn from Daniel Rhodes' book "Clay and Glazes for the Potter"
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© Duncan Shearer, 2005
