A fritt is a melt or compounded glass and the purpose of it is to permit the use of certain ingredients which are not available in the raw state. As glazes are ground in water it is essential that the substances used be insoluble. This condition would prohibit advantage being taken of borax, boric acid, and soda ash, if it were not for the possibility of rendering these insoluble by the operation of fritting.
The following is an example of a fritted glaze:
| PbO Lead Oxide | .30 | } | |||
| ZnO Zinc Oxide | .15 | } | Al2O3 Alumina .15 | } | |
| CaO Lime | .25 | } | B2O3 Boric Acid .40 | } | SiO2 Silica 2.65 |
| Na2O Soda | .20 | } | |||
| K2O Potash | .10 | } |
This will be produced in accordance with the usual calculation by the mix:
| White Lead | .3 × 258 | = | 77 |
| Zinc Oxide | .15 × 81 | = | 12 |
| Whiting | .25 × 100 | = | 25 |
| Borax | .20 × 382 | = | 76 |
| Feldspar | .10 × 557 | = | 56 |
| Kaolin | .05 × 258 | = | 13 |
| Flint | 1.95 × 60 | = | 117 |
The borax contains the required amount of both soda and boric acid and the potash is supplied by the feldspar. Borax, being soluble, must be melted with certain other ingredients into an insoluble glass, thus:
| Fritt: | ||
| Borax | 76 x 2 = | 152 |
| Whiting | 25 x 2 = | 50 |
| Feldspar | 30 x 2 = | 60 |
| Flint | 50 x 2 = | 100 |
| —— | ||
| 362 |
These ingredients are weighed out in double quantity to guard against loss in melting and are fused either in the kiln or in a special furnace. A good fritting furnace is the No. 15, made by the Buffalo Dental Manufacturing Company. The charge is put into a plumbago crucible and when melted is poured out into water. This breaks up the fritt and renders it easy to grind. A similar crucible may be used in the kiln but as the fritt becomes very hard when cold and a crucible must be broken each time, the furnace method is better. If the fritt as given prove too sluggish to pour freely, the feldspar may be omitted, being added, of course, to the glaze mix. The melted weight of the fritt must now be calculated.
Borax contains in each equivalent 180 parts water. Whiting contains in each equivalent 44 parts carbonic acid. Both water and carbonic acid pass off in the melting, thus the 76 parts of borax will be reduced in weight to 40 parts, and the 25 parts of whiting will be reduced to 14 parts. Spar and flint undergo no loss. The fritt after melting will therefore be:
| Borax | 40 |
| Whiting | 14 |
| Spar | 30 |
| Flint | 50 |
| —— | |
| 134 |