Binders harden ceramic powders as they dry. They enable the use of less plastic material mixes that can still adhere and be dry durable.
A classic ceramic glaze (a water based slurry of feldspar, clay and silica powders) can can be applied to the surface of porous ceramic bisque (or even leather hard or dry ware) and it will dry durable enough to be able to handle the ware for further processing. Yet the glaze layer can to washed off in seconds under running water. This ability is a product solely of the binding and hardening properties of the minute clay particles in the recipes. However, in many applications (e.g. where the powder contains little plastic clay or high dry durability or surface adherence are needed), a binder must be employed. These are often organic.
Binders for both bodies and glazes are designed to harden and strengthen these as they dry. They enable excellent adherence to the surface on which they are applied. And ceramic powders containing binders can exhibit remarkable durability on drying. Binders are especially important in glazes, bonding them to even dense bisque surfaces. Binders in enamels can even bond them to metal. The mechanism of a binder can be as simple as a glue that hardens and adheres particles to each other (and any surface they touch). Other binders, especially inorganics, have more complex mechanisms. Body binders make it possible to form powders that would not otherwise be plastic enough to hold a shape (e.g. dust pressing is a common method for these). In fact, bodies having zero clay content (e.g. dental porcelain) can still be hardened using a binder. Glaze binders make it possible to use slurries with very low plastic clay content (either no clay or highly processed clay materials that have little binding power). Conversely, binders can be so effective that a high-shrinkage clay that would normally shrink, crack and fall off a bisque surface on drying can be made multi-layerable on that same surface by the addition of CMC gum.
Binders come at a cost. They can be very expensive. They slow down drying. They make slurries messy and difficult to process and handle. They often pose issues related to CO2 they generate as they decompose during firing. In some industries, like tile, they are avoided where possible, or carefully chosen and monitored (e.g. sodium silicate is used as a binder). Binders are needed to adhere glazes to highly dense, already-vitrified bone china ware. Binders used in enamelling are critical to the process of adhering a glaze to a metal surface. But ware must be fired quickly to avoid oxidation of unglazed parts of pieces. In some cases it is even necessary to fire ware in vacuum furnaces to maximize the ability of the melt to shed the bubbles. Non-organic binders are available (e.g. highly processed smectites, hectorites, bentonites), but thorough testing is needed optimize the percentage and adapt to the issues.
Orton cones are a good example of the use of a binder. Cones are made from ceramic materials, yet resist re-wetting when immersed in water. Once they do slake, the slurry is very sticky and very slow to dewater on a plaster bat. Another common example are prepared commercial hobby and pottery glazes. The binder dramatically slows drying, but they have very good brushing properties. It also enables them to adhere to even already-fired glazes or non-porous surfaces. And the dried surface is hard and difficult to remove, even with water.
The world of binders is typically outside the scope of what a typical potter needs to know. Potters are able to use blends of natural materials and minerals to make their bodies and glazes (using age old processes) and they adapt their methods to the idiosyncrasies these materials present. Industry, on the other hand, needs to optimize processes, so binders are much more applicable.
Binders for Ceramic Bodies
An overview of the major types of organic and inorganic binders used in various different ceramic industries.
The green strength of clay bodies is an important property, it makes them resistant to breakage or damage during handling in production.