What are Pottery underglazes?
An underglaze is used to decorate ceramic bisque or greenware before glazing. Because the glaze will eventually cover it, such decoration is fully durable, and it also enables the manufacturing of pottery and ceramics with a consistent gloss. Pottery underglaze paints come in a variety of brilliant colours and product sizes.
Underglaze is typically administered in a fluid state normally applied to the surface of pottery prior to the addition of any glazing. It’s engineered to endure the high temperatures associated with kiln firing. Broadly speaking, underglazes may include various substances such as slips, engobes, and stains, in addition to products specifically marketed as underglazes.
Distinct from conventional paints, underglazes are tailored to create detailed designs and patterns on pottery, which become visible beneath a layer of transparent glaze. These pigments maintain the surface texture while altering colouration. Underglazes are my favourite choice to enhance existing textures, adding depth and dimension through various application techniques and colour layering. In addition to liquid forms, there is an expanding range of products for applying underglaze, including pencils, crayons, stamp pads, transfers, and semi-moist pads, Â adding versatility to artistic expression.
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Understanding the Various Forms of Underglaze
Underglaze is offered in multiple forms, each suited to different artistic techniques:
- Liquid: Available in vibrant colours, this form comes in containers ideal for brushing and spraying applications.
- Pencils: These are wax-based colouring tools perfect for adding intricate details. The wax dissipates upon firing.
- Chalks: Utilised as dry blocks, chalks can be applied by rubbing onto surfaces to achieve unique textures.
- Slips: Characterised by their high fluidity, slips are excellent for techniques such as dipping, trailing, and pouring.
Commercial Underglazes
Historically, commercial underglazes were formulated similarly to highly pigmented coloured slips, consisting of raw pigment combined with clay and water. Traditionally, these slips were applied to ceramic surfaces when they were either wet or at the leather-hard stage prior to the initial bisque firing.
In contemporary ceramics, most underglazes available on the market have evolved in their composition and resemble that of engobes. Modern formulations incorporate fritted materials that mitigate shrinkage and enable application not only on greenware but also on bisque-fired pottery. This flexibility enhances utility across different stages of the ceramic-making process.
Furthermore, there has been an expansion in the variety of tools for applying underglazes. Options now extend beyond traditional liquid forms to include pencils, crayons, chalks, and semi-moist pads, which offer artists a range akin to utilising watercolours. These developments provide ceramists with diverse mediums to express their artistic visions effectively.
Exploring the Multifaceted Applications of Underglaze
Underglaze has established itself as an indispensable component in the array of resources available to potters due to its versatile nature. Its range of applications includes:
- Applying detailed colouration to leather-hard clay
- Crafting vivid patterns on bisque prior to glazing
- Performing impactful over-glaze brushwork
- Creating sharp, personalised decals
- Employing techniques for accentuating carved sections
- Layering hues and textures beneath glazes
- Generating specialised effects such as lace drapes or resist patterns.
With its extensive variety of colours and forms, underglaze serves as a powerful catalyst for artistic expression.
What Is the Chemical Composition of Underglaze for Pottery?
The composition of underglaze is formulated with specific components, each serving a critical role:
- Flux: This ingredient enhances the adhesion and overall durability of the glaze. Typical fluxes utilised include borax or low-temperature frits.
- Pigments: These are primarily metallic oxides that impart colour to the glaze, with common options being cobalt, copper, and chrome.
- Fillers: Additives such as silica or alumina are incorporated to modify texture and improve sheen.
- Binder: Agents like gum or methyl cellulose are employed to aid in the application process.
- Water: Water is used to increase fluidity, and it facilitates smoother application.
During the firing process, these elements merge effectively, allowing for deep penetration of colours into the clay body and ensuring both aesthetic appeal and structural integrity.
The Use of Gum as a Binder
Typically, when a liquid containing clay is applied to bisqueware, it tends to flake off during the drying process. This phenomenon occurs because as the clay dries and shrinks, the rigid bisqueware does not compress accordingly.
This characteristic can complicate the process of applying coloured clay slip to bisque pottery, as the slip is prone to detaching from the surface upon drying.
However, underglaze formulations incorporate an additive known as CMC gum—or brushing gum—which effectively mitigates these issues. The inclusion of CMC gum serves multiple purposes: it slows down the evaporation rate of moisture in the underglaze, thereby prolonging its malleable state once applied on dry or bisqueware surfaces. Moreover, it enhances the adhesion properties of underglazes by increasing their stickiness; this ensures that upon application, they remain securely attached to bisque surfaces without cracking during drying phases.
The Use of Frits a Flux
Historically, underglazes were composed of coloured clay slips. In more contemporary practices, however, these underglazes have evolved to include frit—a finely ground ceramic glass. When subjected to the high temperatures of a kiln, frit melts into a liquid glass that solidifies as a glossy surface on the ceramic piece upon cooling.
Summary
In summary, underglaze provides a vivid and versatile colouration option that maintains the inherent qualities of the clay surface. Contrary to glazes that may mask the textures beneath, underglaze amplifies these details without obscuring them, offering the potter an effective means for artistic expression.
Underglazes offer greater versatility in design compared to overglazing techniques. For those producing ceramics commercially, it is advisable to experiment with various layers of underglaze and conduct test firings to achieve proficiency with the final result.
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