Molecular 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"

Limit graphs