The difference between slip and underglaze

What Is The Difference Between Slip and Underglaze? The Facts

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There are many terms in the world of pottery, and if you’re brand new to it, this can prove to be confusing. Slip and underglaze may seem similar on the surface, but you will find they are used in different ways.  This may have left you wondering, what is the difference between slip and underglaze?

Slip and underglaze are both made of a mixture of clay and water.  However, slip contains more clay and can add texture to pottery.  Underglaze contains some glass forming ingredients and behaves a more like glaze.  Also, unlike slip, underglaze keeps its color when fired.  Slip tends to change color.

In spite of these differences, they can be used to complement one another nicely.  First, let’s find out a little bit more about the difference between slip and underglaze before looking at how they work together.

Glazes are used to finish a piece of pottery before it is fired so that it will be protected and enhanced.

But What About Underglaze? Here’s what you need to know:

  • Underglazes are often commercially produced and can be mixed together like paint to form other colors for decorating pottery.
  • Experienced potters can create their own underglaze colors using specific recipes
  • Underglaze can be applied to pottery at any point when it is greenware, or bisque fired.  However, it is easiest to apply to leather hard or bisqueware. 
  • It is matte in nature when not covered by a final glaze
  • Generally, underglaze is not used for adding texture effects because it is smooth in consistency.  However, like most things, there are exceptions to this rule!
  • The color of underglaze glaze typically remains the same color even after it has been fired in the kiln.  However, underglaze colors will often brighten and intensify when clear glazed. 
Marbling Underglaze Techniques

You may also have heard of slips, or slip colors. Many potters use this thinnish liquid to decorate their pieces before firing.

How About Slip? Here’s what you need to know about slip:

  • Although readymade slip can be bought, it is typically made by the potter in the studio and color is added to it
  • Though potters do try to use it on bone dry greenware, this is often not successful.  It is best applied to leather hard clay or wetter clay.
  • It can be mixed to different consistencies, either thinner or thicker, and for this reason, it is useful as a decorative for adding texture.  An example of this is slip trailing.
  • When slip is fired it changes color from the unfired state
  • If it is not covered by a final glaze, it dries to a matte finish, just like underglaze

Slip and underglaze are certainly similar in many ways, but they do feature important differences.

The Difference Between Slip and Underglaze - How Are They Used?

In pottery, slip is essentially a liquefied clay that is thin. You may think it’s much like slurry, but it’s actually of thinner consistency. It’s often said that it has the texture and viscosity of heavy cream.

How Is Slip Used?

Slip can be very useful in pottery. You can leave it the color of the natural clay, or you can add pigmentation, such as oxide additives. Whether it is colored or not, slip can be used to decorate your greenware.

Slip has another purpose too. It can be used for casting clay in molds.  This is called slip casting1.  Slip is poured into a mold and allowed to dry.  As it dries it shrinks away and separates from the mold being used.  Because the consistency of slip can be thin, this is a good way to get smooth, thin-walled pottery.

The consistency of slip also makes it useful to use as a gluing agent on your pottery. Slip is used to join two or more pieces of pottery together using the slip and score method.  This is best used on clay that is no drier than leather hard clay.  It can cause problems when attempted on bone dry clay.

difference between slip and underglaze

With slip and score the two surfaces that need to be joined are scored using a pottery tool.  This creates a rough surface.  Slip is then applied to both rough surfaces, and the clay is pressed together.  The scored surface and slip mix together to create a key that will bond the pieces together. 

Here are some other great ways you can use slip for decorative purposes:

Slip Trailing

Due to the nature of slip, it can easily be used to decorate a piece of pottery in a wide range of ways. One way is slip trailing. 

This involves filling a rubber slip trailing bulb the size of your hand with slip.  When the rubber bulb is squeezed, the slip is dispensed onto your pottery through a narrow nozzle on the bulb.  Elaborate textured patterns can be created by slip trailing.

Free Hand Patterns

Alternatively, you can use an object like a comb and drag it through slip to produce patterns on your pottery with dynamism and movement.

Unique Patterns

One variation of slip trailing is to use the force of gravity. If your Slip is applied thinly enough, you can gently tilt the piece and swirl it into any unique pattern that you like.

This process is similar to how painters apply ink to canvas and then blow it with a straw or use gravity to move it around into patterns on the surface.

How is Underglaze Used?

Underglaze is also used for decorative purposes. It is typically used to create colored designs on either bisque or leather-hard clay before being glazed with a final glaze layer and fired in the kiln. Importantly, unlike slip, it retains the same color throughout. This provides more predictable results and striking designs.

Here are some tips and techniques on using underglaze on your pottery:

Make Sure You Experiment

One of the great things about underglazes is that they are available in so many different colors. In this way, it’s much like a painter choosing a color palette for their next canvas.

One thing you can try when you’re confident is to mix different underglazes together to create different colors. Try mixing primary colors and see how many variations of secondary colors you can make.

Try Out Layering

Underglazes can also be layered on your pottery.  The more layers you use the more opaque your color will be. This process will build up color and depth and add new dimensions to your piece.

A single glaze layer still has a charm about it, and this muted color may suit your piece.  In fact, using thinner colors is a way of achieving a watercolor look using underglaze. But if you want to really experiment, there’s nothing like getting into layering and optical color mixing.

Try Out Different Underglaze Techniques

One thing you’ll notice is that underglazes come in different mediums. You can buy liquid underglaze for painting and dipping.  But you can also buy underglaze pens, pencils and chalks. 

This means that you can achieve a wide variety of drawing effects with underglaze too.  It also means you can use underglaze to apply very fine details to your work.  You can literally sketch your designs right onto the piece.

You can also try applying the liquid form of underglaze differently. Try splattering it onto the clay surface with a toothbrush by flicking it. Or try dabbing it on with a sea sponge instead. You can really use anything to apply it to the surface to provide a wide range of textures and effects.

Given the Difference Between Slip and Underglaze, Can They Be Used Together?

You can use both slip and underglaze on the same piece, and they can be used to complement one another.  It’s good to bear in mind that they can behave differently when applied to a piece of pottery.  One key difference between slip and underglaze being that underglaze adds more texture. 

However, these differences can be used to great creative effect.  One technique, for example, is to use slip to create a raised texture on the pottery.  This is allowed to dry and harden.  Then underglaze is applied over the top. 

When the underglaze is dry, the surface of the texture can be sanded off.  This allows the color of the clay body to peep through the underglaze in the pattern created by the underglaze. 

It’s important to remember when sanding bone dry clay to use a respirator mask.  Sanding clay creates a lot of dust which is bad for your lungs.         

Final Thoughts About The Difference Between Slip And Underglaze

Pottery is a truly fun adventure with unlimited scope for creative expression.  When you understand the difference between slip and underglaze, you can really start to experiment in your pottery practice. Just remember to take it slowly, step by step, and get to know the slip and underglaze behave.  Then you can let your imagination shine. 

What you’ll find is that your pottery practice will improve in a short time as you learn how to handle both of these widely used pottery materials. Then you can have a whole lot of fun by trying out different things.  

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Pottery Tips from the Pottery Wheel

Lesley

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Pottery Tips from the Pottery Wheel

I’m Lesley Milne, the creator of The Pottery Wheel.  Like many people, I used the potter’s wheel at school.  But then I began to focus on clay sculpture and I left the wheel behind.  However, more recently, I found myself being drawn back to pottery and the potters wheel.  And so, I have tried to pick up where I left off all those years ago at school. This blog is a chronicle of what I have learned as I got back into the potters saddle!

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