underglaze smudging and bleeding

Underglaze Smudging and Bleeding – How To Prevent It

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Underglaze decoration is incredibly versatile and when combined with a clear glaze can create beautiful results.  I have found that after applying underglaze with painstaking care, it can smudge and run easily.  So, I decided to find out if there was a way I could stop underglaze smudging.   This is what I found out….

Underglaze can smudge when it’s being handled before glazing, whilst the glaze is being applied, or in the kiln.  You can prevent smudging by applying a range of fixatives over the underglaze or applying your glaze in a particular way.  You can also re-fire decorated ceramics to bisque before glazing.  

It’s exasperating to work hard at perfecting your underglaze technique, only for the details of your work to be lost.  Underglaze smudging, bleeding and running can occur for a few reasons, here’s a look at the most common culprits….

What Causes Underglaze Smudging and How to Avoid it

There are a few circumstances in which underglaze will smudge, bleed or run.  They are:

Handling Your Underglazed Pottery Before it Has Been Fired

In its raw unfired state underglaze is very fragile.  If you think about it, underglaze is mainly clay particles suspended in liquid. 

Modern underglazes have been formulated to contain ingredients that melt in the kiln too.  Nevertheless, they still have a high percentage of clay.  They are basically colored slips with added ingredients.

What is often said about bone dry greenware is that it is incredibly fragile.  It needs to be handled carefully as it can break very easily.

what to do whilst waiting for your kiln to cool down

Well, the same is true of unfired underglaze.  Underglaze that has been applied to your pottery and left to dry is basically bone dry clay.  As such it is apt to crumble and break if it is not treated with great care. 

In addition to this, underglaze is sometimes applied in ways that increase its fragility.  An example of this is spraying or airbrushing underglaze onto your pottery.  When you apply underglaze with an airbrush, you are basically misting your underglaze onto the pottery.  As such, when it dries, it creates a fine powdery layer that can be rubbed off effortlessly.

In this state, underglaze can be damaged easily as potters move their ceramics around.  Or as the pottery is being transferred into the kiln.

How to Prevent Underglaze Smudging Before it Has Been Fired

Here are some inventive solutions that potters typically use to protect their unfired underglaze: 

Spray on Starch

One solution is to spray your unfired underglaze with spray-on starch.  This is not a new-fangled ceramic product.  It is simply the kind of spray-on laundry starch that you can buy in a grocery store.

The best way to apply the starch is to put your pottery (carefully!) onto a banding wheel.  Then, gently rotate the banding wheel and mist your pottery with the spray-on starch.

This can be done with underglaze that has been applied to greenware or to bisque fired ceramics.

Once the starch has dried, it will form a hard protective layer over the underglaze.  This trick is often used by potters who want to transport their pottery from A to B. 

Often in the early days, before you have your own kiln, you can find yourself driving your pottery around.  Most new potters will take their pottery to a local kiln to have their pottery fired.  Applying some spray-on laundry starch to your underglaze can help to make the journey a little perilous.

underglaze smudging and bleeding

Hair Spray

Hair spray is another way to make your journeys a little less hair-raising (if you will pardon the pun).

As an alternative to spray starch, potters will often use hair spray in the same way. Hair spray is cheap and readily available, and like starch will hold the powdery clay particles in place.

When I first heard of this, I was worried that my pottery might turn into a fireball in the kiln.  Hair spray is notoriously flammable.  However, in practice, the hair spray simply burns off in the kiln.  No fireballs involved.  It’s a simple way to stop your underglaze smudging.

Artist’s Fixative

Another way to stop your underglaze smudging when you handle it is to use an artist’s fixative.  These are generally produced for artists to spray on chalks, pastels, and charcoal, all of which smudge. 

However, some are suitable for being used on underglaze.  It is sometimes a little unclear from the label if they can be used on ceramics.  So you may need to experiment a little. 

Nevertheless, some brands specifically state that they are suitable for underglaze.  An example of this is Sennelier’s d’artigny Oil Pastel Fixative.

Mixing Liquid Starch Into Your Underglaze

Yet another solution1 is to mix liquid starch like Sta-Flo directly in with the underglaze.  Liquid starch is sometimes included by potters in their underglaze recipe to improve the underglazes brushability.  The starch makes the underglaze stiffer. 

However, it also has the added bonus of making the glaze harden when it dries, binding in the clay and stain particles.  

Because laundry starch is made from products like rice, corn, and wheat, it can break down and go moldy.  Most commercially produced starches contain a detergent that stops mold growth. 

However, it’s a good idea to mix small quantities of your underglaze with starch, in case mold does grow.  You don’t want your favorite underglaze to be beset with mold.  It isn’t the end of the world if it does go a bit moldy.  You can just scoop the mold off and use it as intended.

Underglaze Smudging and Bleeding When Glaze is Applied

Underglaze can be very beautiful.  However, it does not make pottery non-porous.  For this reason, it’s very common practice to apply a clear glaze on top of underglaze decoration.

If you have applied your underglaze to greenware and then bisque fired it, you are in the clear.  Underglaze that has been fired will not smudge, bleed or run when it is glazed.

However, underglaze can be applied to bisque ceramics and then glazed before being fired again.  Also, some potters like to do single firing, where glaze is applied to greenware.  In a single firing situation, it’s likely that glaze will be applied directly on top of unfired underglaze.

The trouble is that when you apply glaze to unfired underglaze, the underglaze can bleed and run.  The reason for this is simple.  When the liquid glaze is applied, the unfired particles of underglaze dissolve into the glaze suspension.

This can result in a few things.  If you are using a brush to apply your underglaze, it can cause streaking.  In effect, the glaze on the brush is dragging the underglaze across your pottery. 

Alternatively, if you are dipping your pottery in glaze, the underglaze can simply bleed.  If this happens the design can become fuzzy and blurred.  However, there are a few things you can do to avoid underglaze smudging in this way.

How to Avoid Underglaze Smudging When Applying Glaze

  • Apply the first layer of underglaze carefully with a sponge.  It’s also a good idea to dilute it down a little.  Apply the underglaze thinly so that it dries quickly.  This gives the underglaze particles less liquid to spread into.

    Once this layer has dried and sealed your underglaze, you can apply another couple of coats as you would normally. 
underglaze smudging and bleeding
This clear glaze goes on green and is clear when fired.
  • You can also use some of the sealing techniques suggested above to stop underglaze smudging when it’s being handled. A layer of starch, hairspray, or fixative between the underglaze and glaze will not have any adverse effect in the kiln.  The sprayed-on layer will simply burn off during the firing process. 

  • If you do want to use dipping glaze rather than brushing or sponging, then dip your ceramics gently.  Dip the glaze and draw it out again carefully.  Don’t wiggle the pot around when it’s submerged in the glaze mix as this will drag on the underglaze.

  • When you are letting the glaze dry, don’t let the pottery sit in one position whilst it dries.  Instead, move the pottery around gently and change its orientation. You can do this by gently moving the pottery through the air with tongs.  This will prevent the glaze from settling and pulling in one consistent direction as it dries.

  • Make sure that your bisque ware is not too cold.  If your ceramics are at room temperature, or a little warmer then the glaze will dry quicker.  The quicker it dries the less chance the underglaze will have of smudging or bleeding.
underglaze smudging and bleeding

Underglaze Smudging and Bleeding Whilst Being Fired

It is easy to tell if your underglaze is smudging when you’re handling it or applying glaze.  However, if your underglaze is bleeding or running whilst it is being fired, the cause may need more investigation. 

There are a few factors that can cause underglaze to bleed in the kiln.  So, it may be a matter of trial and error to find out which one is the problem for your pottery.  Sometimes, it will take a while to alter certain factors when you are firing to identify where the problem lies.

Reasons Underglaze Smudges and Bleeds When Being Fired and What To Do

1) Underglaze or Glaze Becoming Too Liquid / Fluid

Some underglazes become more viscous when they melt in the kiln. The same is true of glazes.  How liquid, or fluid, an underglaze or glaze becomes is referred to as ‘melt fluidity2’.

If an underglaze becomes very fluid when it melts, then it is more likely to bleed.  Certain colors like cobalt blue have a high ‘melt fluidity’ and are more likely to bleed and run when fired. 

Likewise, you may have used a clear glaze over your underglaze that becomes very fluid when it’s fired.  If your underglaze is covered by a very viscous glaze, then there is a greater chance the underglaze will bleed.

Some experienced potters might feel confident trying to adjust how fluid an underglaze becomes.  This can be done by tweaking the ingredients in an underglaze.  However, most of us accept the fact that of the underglazes that we buy, some will bleed more than others.

If you find that your underglaze is running or bleeding, then it’s worth simply swapping out the brand you use.  You may find that another brand will be less viscous when fired, and your problem will be solved.

It may be that the problem lies with the melt fluidity of the glaze applied on top.  So, switching to another glaze and testing that might help identify the cause of the bleeding.

2) Applying Your Underglaze Too Thickly

Applying your underglaze too thickly can cause it to flake off or bleed when it’s fired.  This is a tricky one to gauge because achieving brightness or opacity in underglaze requires layering. 

The solution to the question of whether your underglaze is too thick may again be one of trial and error.  Do a few test pieces with underglaze of different thicknesses and see how they respond.  Either use fewer layers or water your underglaze down and see if that makes a difference.

The other option is to fire your underglaze and then apply the glaze over-fired decoration.  You can either apply your underglaze to greenware and then bisque fire.  Or if you want to use the underglaze on bisque, then you can re-fire your painted bisque ware before glazing.

3) Applying Your Glaze Too Thickly

Are you applying your clear glaze too thickly?  This can also cause underglaze decoration to bleed and smudge.  

Try thinning down your glaze with a little water, so that the glaze layer is not too dense.   

Also, make sure the glaze is thoroughly mixed before application.  This will help ensure that no patches of glaze on your pottery are thicker than any other.

4) Firing Too Hot

Are you firing your underglaze to the right temperature?  Underglaze like clay and glazes have an ideal temperature range that they perform at. 

Check that your kiln is not getting too hot for the underglaze that you are using.  If you overfire underglaze it can smudge, bleed and lose its pigment.

5) Firing Too Quickly

Try firing your ceramics more slowly.  Sometimes underglaze bleeding happens because the pottery has been fired too quickly.  If the kiln is ramping too fast, the underglaze, glaze, and pottery don’t have time to mature properly.

6) Are You Soaking For Long Enough?

In addition to this make sure that the pottery soaks long enough in the kiln.  Soaking occurs in the last part of the firing schedule before the kiln starts to cool down.  It involves keeping the kiln at its highest temperature for a while. 

Potters vary in how long they leave their ceramics to soak.  Some recommend around 20 minutes.  Others will leave the kiln to soak for up to an hour.  You may need to experiment with what works for you. 

Soaking gives the clay, glaze, and underglaze a chance to fully mature.  A long enough soak will help rectify imperfections in the glaze and smooth it out.  As such, it may reduce underglaze smudging and bleeding.

Final Thoughts

Underglaze smudging and bleeding is a frustrating problems. After making your pottery and decorating it, it’s a pest to take it from the kiln and find the underglaze has run. Hopefully this article will have given you some tips on how to avoid it. And help you decorate your pottery with nice crisp underglaze edges.

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References:
  1. Amazing Glaze: Techniques, Recipes, Finishing and Firing‘.  By Gabriel Kline.
  2. https://digitalfire.com/4sight/glossary/glossary_bleeding_colors.htm

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