Is Colored Slip the Same As Underglaze?

a picture of underglaze and slip mugs

Pottery is a popular hobby, and because of that, Potters have developed a number of materials and mediums to decorate clay pieces. Among these, colored slips and underglazes are quite common. At first look, both these materials can look pretty similar.

But is colored Slip the same as Underglaze? They are very similar in that both of them are a liquid clay mixture with added colorants like mason stains. The critical difference is the amount of clay and colorant and frit (Frit is silica and flux). Colored slips contain higher clay content and no frit. Underglazes contain less clay content and a small amount of frit.

Now that you know the basic trait that distinguishes the two let’s take a detailed look at their differences and ways of application.

Differences Between Underglaze and Colored Slip

a picture of colored slip and underglaze

Texture

The main difference in slips and underglazes is the texture. Underglazes don’t have a texture or thickness as they contain less amount of clay. Colored slips, however, do leave behind texture and some thickness because they contain more clay. The amount of texture depends on how smooth and thick the clay is also if there is any sand or grog in your Slip.

The thickness of colored slips is found during the application process. Colored slips are typically applied on leather-hard clay, but can also be applied to clay at any stage depending on your skill level. The colored Slip drawn on a piece is raised, also carving over colored Slip is very popular.

Underglazes are meant for all the different surfaces, while colored slips can also be used at all stages, but this can be a little trickier because of the higher clay content in Slip and the shrinkage level.

Color

The next key difference is found in colors. Underglazes have a large range of colors, which can be mixed easily. Commercial colored slips have a smaller range of colors as compared to underglazes. I made a step by step video on How to Make Colored Slip you can find it HERE, or if you just want to make a Plain Slip I have a video for that also, you can find it HERE.

To mix colored slips, add your own watered-down thinned clay and colorant like mason satins. Only compatible slips should be mixed together, i.e., Colored Slips that are made for specific shrinkage rates and firing temperatures. 

Additionally, Underglazes are available in the form of pens, crayons, and pencils along with the liquid mixture, while colored slips are not. Underglazes can give greater detail because of the less clay content and a small amount of frit.

Firing

Underglazes and slips stay in one place when fired.

Underglazes can have a specific firing temperature, so check the firing range before use.

Colored slips do have a specific firing temperature. Firing above that temperature will result in the Slip being over-fired, resulting in bubbling and even melting. Always know the firing temperature of the clay content in your Slip.

Moreover, colored slips change their color during firing. The color comes out more after it is fired. You may think you didn’t add enough colorant, but keep in mind the color will come out more after it has been fired. Underglazes also come out more vibrant.

How to Apply Underglaze

a picture of painting with Painting with underglaze

Underglaze is mainly used for decorative purposes. It’s generally used to create colored patterns on clay. Underglaze can be applied in all stages of pottery-making: On greenware – from wet, leather-hard, and bone-dry to bisque- ware. You could add more or less water depending on the condition of clay. 

Once the Underglaze has dried, a top layer of clear glaze is coated onto the pottery before being fired in the kiln to give a glossy finish and make the color permanent. Underglazes always hold on to their colors in the kiln. Their result is very predictable, and the designs are distinguishable.

Here are some valuable tips on applying underglazes:

  • Once you’ve gotten the hang of Underglazing, try mixing different colors together. Mix primary colors like red, blue, yellow, white, and black together and see how many color variations and tones of these colors and secondary colors you can come up with.
  • Underglazes should be layered if you want the result to pop out once the piece has been fired. The more layers you use, the less translucent your color will be. Building up color also adds depth and dimension to a piece.
  • You can use a single underglaze layer to suit a muted piece. Muted colors are great if you want the color of clay to be more prominent.
  • Using a thin layer of Underglaze with a brush can give a watercolor effect. You can also get a drip effect by using slightly thinner Underglaze.
  • Underglazes can be dipped in, painted on by a brush, or drawn with underglaze pens, pencils, and crayons. This lets you add fine details. You can also use Underglaze to Bubble Glaze, dip a String in the Underglaze to create a pattern, Scraffito, Mishima, Transfers, and Sponge Glazing.

How to Apply Colored Slip

a picture of a gnome painted with slip and underglaze

Colored slips also contain pigment like underglazes and are generally used for decorative purposes.

Slips can be applied on all stages of clay, but remember to know the type of clay you are using for your colored Slip. When decorating, it’s best to use the same type of clay. This makes your slip and clay compatible giving it the same shrinkage rate. That’s why it’s best to make your own Slip.

Here are a few tips on using slips:

  • You can use slips as a gluing agent between two pottery pieces. It’s best if you use the same type of clay with the same shrinkage rate.
  • Slip can be trailed on a pottery piece by a slip trailer, which is a small squeezable bottle with a nozzle. The Slip gives your piece wonderful results. You can use it to make elaborate patterns too.
  • Use a brush to paint on the colored Slip. You can either draw freehand or use a stencil.
  • You can use colored slips in a unique way by applying it thick or thin. Once you’ve applied it, pick your piece up and swirl it around to create a pattern. Gravity plays an important role in this method of application. 

How to Use Underglaze and Colored Slip Together

Both Colored Slips and Underglaze can be used on one pottery piece, and they can be applied to obtain various complimenting textures and colors.

The biggest difference is that Slip is made up of clay and colorant and will add more texture through the medium it’s worked with. Underglaze is made up of Clay, Colorant, and Frit and can have a smoother texture.

This difference in nature can be used in a creative way. You can use a colored slip to create an interesting pattern. The area will rise and have texture. You can use a thick or thinner Slip to create different patterns. After the Slip is totally dry, you can apply an underglaze on top.

There are so many different ways you can decorate with Slip and Underglaze. Have some fun with it. Experiment and see what beautiful decorations you can come up with. Just make sure your Slip, Underglaze, and Clay are compatible and the same firing temperature.

Conclusion

Colored slips and Underglazes may seem like two different names for the same material, but they are different. Underglazes are easier to apply. They are better for more detailed designs and beginner-friendly. While colored slips are just as much fun to decorate with once you get the hang of it.

Choosing between the two comes down to your needs. A good rule of thumb is to always check the label of your, Clay, Slip, and Underglaze for the Firing or Cone temperature. If it does not say the firing temperature do not buy it. With that said, have fun Underglazing and Slip decorating. 

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