Underglaze, as the name suggests, is designed to be applied before pottery is glazed. 

Pottery glaze has two main functions.  Its functions are to decorate the pot and also to create a non-porous glassy seal on the piece.  By contrast, underglaze is really intended to be used to decorate pottery rather than seal it. 

If you want to know more about the difference between glaze and underglaze, check out this article on underglazing.

Underglaze

Underglaze comes in different forms.  Probably the most common way to use underglaze is in its liquid form. 

Liquid underglaze is very versatile and can be used a lot like many water-based paints. 

For example, it can be painted onto pottery using liquid underglazes like these.  When it’s applied as a paint, it can be used to create a watercolor wash effect.  Or it can create dense rich colors by applying several thicker layers of underglaze.

Alternatively, underglaze can be sprayed onto pottery.  The effect that is achieved from spraying depends on the type of equipment you are using.  If you spray using an airbrush, then you can achieve a fine-looking gradient. 

However, you can also spray underglaze with an atomizer.  This is a simple gadget that is operated manually by blowing through a pipe to spray the underglaze across your pottery.  This creates a spray made up of different-sized droplets and can look lovely. 

Applying Underglaze

The beauty of underglaze is that it can be applied to clay at any stage.  It can be painted onto leather hard clay, bone dry clay or bisque fired pottery. 

If it’s applied to leather hard clay, you can use underglaze to highlight carved features in your pottery.  This decorating pottery technique is called sgraffito. ThePotteryWheel.com has a link to help in understanding this process with sgraffito - look here.

One way to make sgraffito is to paint leather hard pottery with underglaze.  Then when the underglaze has dried, you can carve a design into the clay surface.  When you carve, the clay underneath is revealed and contrasts with the underglaze colors on the surface of the pot.

Different Types of Underglaze Medium

Whilst underglaze is often applied as a liquid, there are other really great underglaze mediums. 

You can buy underglaze crayons, pastels, chalks, and pencils.  If you like drawing these can be used to decorate pottery with your drawing skills.

Underglaze Chalk Crayons – view on Blick Different underglaze mediums can be combined. 

For example, you might paint on an underglaze design with a brush.  Then use underglaze pencils or pastels to highlight areas on your design.  Really, you can combine them in whatever way you choose.   

Reference -  thepotterywheel.com -