Introduction to Glaze Calculation

ceramicswiki · May 18, 2017, 3:43am

Introduction

A glaze calculation works out the chemicals a particular glaze recipe contains. You can take all the materials that make up a glaze, break them into their constituent parts and show them in proportion to one another. This is rather like the nutrition information on a food packet - it shows what the recipe contains rather than the ingredients that made it. Where food talks about carbohydrates, proteins, vitamins and minerals, glazes talk about silica, alumina, potassium and a host of other chemicals that produce the end result.

This information is useful when trying to reformulate a glaze to use different materials, such as swapping one frit for another, or using a local feldspar. It is also useful when trying to understand why a glaze behaves the way it does or to predict the way an untested glaze will look.

Linda Arbuckle explains it like this:

The purpose of glaze calculation is to determine the total amount of each element present in a glaze, and the proportions relative to each other. With that information at hand, it is possible to calculate materials substitutions, revise melting points, and do other useful calculations. In the end, testing is the final proof, but unity molecular formula gives you a more informed method of choosing what to test.

What makes a glaze?

Glazes may be thought of as a colorless glass, consisting of:

glassformer - usually silica
viscosity agent - often alumina
flux (something that lowers the melting point) - Sodium, Calcium, Magnesium and many more

Some glaze materials are added in their pure form, such as lithium carbonate. However, many glaze materials have a more complex composition. These are classified as composite materials and compounds.

A compound is a natural material that is made of a combination of differing elements. For example, Potash Feldspar is a combination of K2O (Potassium Oxide), Al2O3 (Aluminium Oxide) and SiO2 (Silicon Dioxide). Kaolin also falls into this category as does Gerstley Borate, Nepheline Syenite, Spodumene and many more.
A Composite material is a man made material that combines more than one element, such a a frit, clay body or pre-mixed glaze.

Oxides

Composite Materials & Compounds are made up of oxides joined in lattices.
TODO: Explain oxides.

Molecular mass of common formulas

Moved to https://wiki.glazy.org/t/glaze-calculation-in-software/

Materials

Here are four basic materials that can be combined to make a glaze:

Silica is 100% SiO2
Potash Feldspar is 64.8% SiO2, 18% Al2O3, and 17% K2O
Whiting is 56.1% CaO
Kaolin is 47.3 SiO2, 40.2 Al2O3

Why don’t the numbers add up to 100%?

Because component elements like carbon in carbonate materials burn out, volatilise or materials revert to their basic oxide form which gives off gasses during firing, this is classed as Loss On Ignition (LOI).

Here are the materials again, this time with Loss On Ignition (LOI) included:

Material	SiO₂	Al₂O₃	CaO	K₂O	KNaO	LOI	Total
Silica	100%					0	100
Potash Feldspar	64.76%	18.32%		16.92%	16.92%		100
Whiting			56.10%			43.90	100
Kaolin	47.29%	40.21%				12.50	100

Theoretical Materials

All material analyses are to some extent theoretical in that they approximate actual real-world materials. The actual materials we use in the studio come from various natural sources and contain impurities and inconsistencies. For example, there are many types of Potash Feldspar, and each type has a different analysis. Even within types of Potash Feldspar, there may be different analyses for different batches of material at different times.

Imagine you are at a grocery store but didn’t know what to cook. So you and find the following recipe via an app on your phone:

Roasted Delight
Ingredients:
1 carrot
1 onion
1 pound of meat
1 teaspoon of salt
1 teaspoon of pepper

What does “1 pound of meat” mean? It could be steak, chicken, or fish, and the final dish would be much different with each type. You see in the recipe reviews that people used fish, but they didn’t mention what type of fish. Freshwater, saltwater?

Just as with our fictional “Roasted Delight” recipe, some older glaze recipes just list “Feldspar” (meat) or “Potash Feldspar” (fish).

A sample of analyses for Custer Feldspar, a type of Potash Feldspar:

Ingredient	Type	Parent	SiO₂	Al₂O₃	MgO	CaO	K₂O	Na₂O	P₂O₅	Fe₂O₃	LOI
Custer Feldspar	Feldspar	Potash Feldspar	68.50	17.00		0.30	10.00	3.00		0.10	0.30
Custer Feldspar (1989)	Feldspar	Potash Feldspar	68.64	17.39		0.30	10.36	3.02			0.30
Custer Feldspar (2000-2012 Ron Roy)	Feldspar	Potash Feldspar	72.40	15.27	0.07	0.27	7.52	3.21	0.20	0.21	0.65
Custer Feldspar (pre-2000)	Feldspar	Potash Feldspar	68.70	17.25	0.05	0.18	10.06	3.04	0.31	0.12	0.66

When working with actual materials in the studio, it’s best to use the most accurate analysis you can find for each material.

Explain KNaO

Glaze Recipes

Imagine the following recipe for Macaroni & Cheese:

Servings: 4
Units: US
Ingredients:

8 ounces elbow macaroni
1⁄4 cup butter
1⁄4 cup flour
1⁄2 teaspoon salt
1 dash black pepper
2 cups milk
2 cups shredded cheddar cheese

Glaze recipes are similar to food recipes.

As an example we will use a commonly referenced recipe, Leach 4321. Leach 4321 is so named because the recipe amounts are easy to remember (40%, 30%, 20%, and 10%). Using the materials from above:

Leach 4321:
Orton cone 9-10, Oxidation or Reduction

Ingredient	Amount
Potash Feldspar	40%
Silica	30%
Whiting	20%
Kaolin	10%
Total	100%

Just as a Macaroni & Cheese recipe would describe how the ingredients should be prepared, glaze recipes also specify both a temperature range (measured by Orton cone) as well as atmosphere type (in this case, the glaze can be fired in either oxidation or reduction). Other preparation notes are commonly added to glaze recipes, such as the best specific gravity and application type (e.g. pouring, spraying).

Benefits of 100% format

From Linda Arbuckle:

The usual convention is to represent a glaze recipe in 100% batch form, with colorants, opacifiers, and conditioning agents (e.g. gums, bentonite, etc.) listed as additions to the basic 100% glaze. Please see the handout on calculating 100% format. Advantages of the 100% format:

glazes are more easily compared in uniform format
calculation of percent additions like colorants easier with 100 unit base

Weight Units

Glaze recipes are not given in any specific weight units. The recipe can be weighed up in any units, as long as the same unit is used throughout, and the proportions of materials will be equivalent and yield the appropriate result.

% Analysis

Show how percentage weight analysis is calculated with Leach 4321
Explain roles of various oxides in Leach 4321

To calculate the percentage analysis for Leach 4321, for each material multiply each oxide by the percentage amount.

First, let’s start with Potash Feldspar. Potash Feldspar’s analysis is:

Ingredient	SiO₂	Al₂O₃	K₂O	LOI
Potash Feldspar	64.76%	18.32%	16.92%	0

For each oxide in Potash Feldspar, multiply by the amount (40%, or .40):

SiO₂ (Silica) is 64.76 x .4 = 25.904
Al₂O₃ (Alumina) is 18.32 x .4 = 7.328
K₂O (Potassium) is 16.92 x .4 = 6.768
Loss On Ignition (LOI) is 0 x .4 = 0

Leach 4321 Percentage analysis for Potash Feldspar

Ingredient	Amount	SiO₂	Al₂O₃	CaO	K₂O	KNaO	LOI
Potash Feldspar	40	25.9	7.33		6.77	6.77

Completed Leach 4321 Percentage Analysis:

Ingredient	Amount	SiO₂	Al₂O₃	CaO	K₂O	KNaO	LOI
Potash Feldspar	40	25.9	7.33		6.77	6.77
Silica	30	30
Whiting	20			11.22			8.78
Kaolin	10	4.73	4.02				1.25
Total	100	60.63	11.35	11.22	6.77	6.77	10.03
100% no LOI		67.39	12.61	12.47	7.52	7.52

Explain mol vs. weight

Mole %

Show how to calculate Mol percent using Leach 4321
Explain differences from % analysis
Explain UMF, Seger, etc.

UMF

For UMF, unify using the following oxides:

Li2O
Na2O
K2O
MgO
CaO
SrO
BaO
ZnO
PbO
FeO
MnO

UMF Unity oxides in various software programs:

Program	Li2O	Na2O	K2O	MgO	CaO	SrO	BaO	ZnO	PbO	FeO	MnO	CuO	CoO	NiO	P2O5	Notes
Desktop Insight	Li2O	Na2O	K2O	MgO	CaO	SrO	BaO	ZnO	PbO						P2O5	I think can be modified?
Matrix	Li2O	Na2O (as KNO)	K2O (as KNO)	MgO	CaO	SrO	BaO	ZnO	PbO							Unity oxides can be changed. Can also include colorants and B2O3.
GlazeMaster	Li2O	Na2O	K2O	MgO	CaO	SrO	BaO	ZnO	PbO	?	?	?	?	?		?
HyperGlaze	Li2O	Na2O	K2O	MgO	CaO	SrO	BaO	ZnO	PbO			CuO	CoO	NiO		Unity oxides can be changed. Can also include colorants and B2O3.
Matt Katz's Spreadsheet	Li2O	Na2O	K2O	MgO	CaO	SrO	BaO	ZnO
Glazy	Li2O	Na2O	K2O	MgO	CaO	SrO	BaO	ZnO	PbO	FeO ?	MnO ?					Unity oxides cannot be changed.

Glaze Limits

Stull

Stull Al/Si chart showing Leach 4321 analysis using different feldspars.

The following Stull chart shows the Leach 4321 recipe with different feldspar and kaolin analyses.

Pieter_Mostert · August 9, 2017, 7:58pm

A couple of nit-picky comments:

Strictly speaking, potassium is the element K, whereas K2O is potassium oxide, or potassia (although I don’t think many people use this word). Not sure if it’s better to always make the distinction, or just call K2O potassium and add a remark that we really mean potassium oxide.
The Stull chart would be simpler if it just had Kaolin and the different Potash Feldspars.

ceramicswiki · August 10, 2017, 2:57am

you’re right, changed the wording. this article still needs a lot of work. might need to separate into separate articles, e.g. UMF, Stull Charts, etc.