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How to Mull Acrylic Paints Efficiently (And With Little Mess)

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I like to work with acrylic paint because it dries quickly and there are many colors and substrates available.

Quickly Prepare Acrylic Paint on the Slab

Many pigments, especially organic pigments, are hydrophobic and are difficult to disperse into water-based paint mediums like acrylic emulsion. The method of paint preparation that I outline consists of the following steps:

  1. Weigh out the desired amount of your dry pigment onto the glass slab.
  2. Apply a wetting agent and water to get a uniform slurry mix.
  3. Stir to work out as many lumps and larger agglomerations as possible.
  4. Put the smooth but somewhat gritty pigment dispersion into a small cup—but leave a 1 gram or so portion on the glass slab.
  5. Use the glass muller to grind the smaller agglomerations of the pigment dispersion until smooth.
  6. Set the fully dispersed pigment into a slightly larger cup and repeat with another 1 gram portion of gritty dispersion until all pigments are smoothly disperse.
  7. Add a few drops of clear ammonia to pigment dispersion just until it has a slight ammonia smell. This indicates an alkaline pH.
  8. Add sufficient acrylic medium to the muller pigment and stir until uniform.

Tools and Materials

These are the items needed to embark on the acrylic paint-making journey:

  • Two metal paint spatulas
  • A dry pigment of your choosing
  • Distilled water
  • Orotan 731K or similar wetting and dispersant
  • Clear and non-sudsy ammonia
  • Acrylic emulsion of your choosing
  • Prepared glass plate
  • Large glass muller
  • Small disposable cups for storing and mixing
  • A cheap roll of paper towels, rags, or torn t-shirts
  • 91% isopropyl rubbing alcohol
Weighing Out Approximately 1 Gram of Pigment Red 254

Weighing Out Approximately 1 Gram of Pigment Red 254

Weigh Out Dry Pigment to Make Dispersion

Once all tools and materials are gathered, it is time to weigh out the amount of dry pigment you will be using. This is the beauty of preparing paint, you only use what you need. Keeping that in mind, I’d recommend a 1 or 2 gram portion of dry pigment. That should be enough for swatch testing and trying it out on a small project.

To the mound of dry pigment in the glass slab, add a few drops of Orotan 731K and enough distilled water to make a liquid of salad dressing consistency. If pigment resist wetting add denatured or rubbing alcohol to assist in pigment wetting.

Orotan 731K for Wetting and Dispersing Hydrophobic Pigments

Orotan 731K for Wetting and Dispersing Hydrophobic Pigments

Wetting and Mixing Pigment With Water

Wetting and Mixing Pigment With Water

Wetting and Dispersing Dry Pigment

The ease at which a pigment wets and disperses depends on whether you have an inorganic or organic pigment. Inorganic pigments have larger particle, less surface area, and are usually water loving or hydrophilic. This makes them easy to disperse just with water and acrylic emulsion but a small amount of Orotan 731 K or similar surfactant mix will make breaking up agglomerations smoother.

With an organic pigment like pyrrole red, a significant amount of dispersion aid will be necessary. Sometimes you might want to add denatured ethyl alcohol or isopropyl rubbing alcohol to help wet the pigment. Both alcohols evaporate pretty quickly so that should not affect the end product. Just make sure the alcoholis gone before adding the acrylic medium.

Mulling to brake agglomerations in pigment dispersion

Mulling to brake agglomerations in pigment dispersion

Disperse with Glass Muller

After thorough wetting and mixing the pigment with a painting knife, it’s time to mull the pigment into a dispersion . Using the glass muller, make circular motions changing directions frequently. A few figure 8 patterns will also help add shear force to break up agglomerations. The breaking down of agglomerations will be observed as gritty grinding noises.

Pigment Dispersion Complete after about 4 Mulling Cycles

Pigment Dispersion Complete after about 4 Mulling Cycles

Add Acrylic Medium

After all agglomerations have been removed from the dispersion, use the pallet knife to add an equal volume of acrylic medium to pigment mound. Stir well until well mixed and all whiteness from acrylic medium is gone. Take a tiny drop and spread on some scrap paper. Look for any speckles or granules in the streaks. If needed mull the paint. But be mindful that acrylic dries quickly and you might end up mulling dried paint into your final product. Do not mull more than 10 minutes to avoid this pitfall.

Adding Acrylic Medium

Adding Acrylic Medium

Stir with Paint Knife.  Your paint is ready!

Stir with Paint Knife. Your paint is ready!

Test By Making Paint Swatches

Paint a full mass-tone swatch on a top corner of sketch bookamd label with pigment name and color Index number if you know it. For example, pyrrole red is ”Pigment Red # 254”. Write that down in case you need to cross reference later. Also make notes of portions or characteristics of the paint to help you with future experimentation.

Then put some water on the brush and rub the remaining paint on other side of sketchbook page to see if pigment is fully dispers.

Paint Swatches in Sketch Book

Paint Swatches in Sketch Book

The Clean Up

If you followed through and did not deviate from the process, you should have a dirty spot on your glass plate only slightly larger than the diameter of the muller. The muller should be easy to clean since you have extracted most of the paint from it. Most of the paint on the spatulas will scrape off with old newspapers or junk mail.

  1. Take a rag or paper towel. Squirt some 91% isopropyl rubbing alcohol on it and clean the spatulas. This should take less than 30 seconds.
  2. Next, squirt some rubbing alcohol onto the pigment stain on the glass plate. Let it soak in for 2 minutes.
  3. Move on to the muller and squirt it with alcohol as you wipe it with your rags. It should come right off. Once all remaining color is removed off the muller, we now move to the glass plate. It’s been soaking in alcohol because it is ever so slightly more difficult to clean.
  4. Start wiping up the alcohol and partially dissolved paint. Pick up what you can in first attempt. Use a clean rag or paper towel and fresh alcohol to make sure all color is removed.
  5. That’s it! Clean up should take less than 10 minutes! This is a critical timesaver if you are preparing a few different colors.

Comments

Jason (author) from Indianapolis, IN. USA on November 18, 2019:

Orotan 731 K is great stuff. For the difficult to wet organic pigments, I still use a dash of denatured or isopropyl alcohol to get started. I usually have to mull the pigment dispersion several times before adding the acrylic medium anyways. So most alcohol is gone by that point. Orotan actually adheres to the pigment particles so they stay dispersed.

Ernie on November 17, 2019:

Thanks Jason! That is very good information. I found your post when searching for Orotan 731 K, as I ordered some recently from Kremer Pigmente to make watercolor paints, using Aquazol as the binder.