Flocculation; viscosity... huh?

For many of you, the terms flocculation and its antonym, deflocculation are going to elicit giggles. They are funny sounding words, but what do they mean and how do they apply to ceramics? The other term I should define up front in this article is 'viscosity'.

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Timing Viscosity

According to the IUPAC definition, flocculation is "a process of contact and adhesion whereby the particles of a dispersion form larger-size clusters". Flocculation is synonymous with agglomeration and coagulation/coalescence.

Viscosity: the state of being thick, sticky, and semifluid in consistency, due to internal friction.
"cooling the fluid raises its viscosity"

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In summary, flocculants like Epsom Salt and CMC Gum make particles stick together, gelling a slurry. Deflocculants like Sodium Silicate and Darvan on the other hand create a charge that makes the particles repel one another, reducing friction which creates a more fluid glaze or casting slip. Viscosity is the resulting 'flow' of any given slurry.

To measure viscosity you will need to get a flow cup. For $10, I recommend getting a Ford #4 cup. The way flow cups work is to time how long it takes the glaze/slip to flow through using a stopwatch. Using a Ford #4, I like glazes to land between 10-12 seconds for spray application, and 8-10 seconds for dip application, but those are personal preferences, there isn't a universal best number to shoot for. Just record your water % and viscosity numbers along with the amount of Epsom Salt or Darvan you used to your recipe card (you only have to do this once).

Weighing Specific Gravity

Now that we have the basic terms identified, I want to talk about one of the pitfalls of introducing these ingredients in your ceramic studio. In my previous blog post, understanding-water.html, I discussed the importance of measuring water in your recipes. For the most part, glaze recipes are simplest when only using water to get the correct consistency  because you only have to measure specific gravity. As soon as you start introducing flocculants or deflocculants to your recipe you have to check both the specific gravity and viscosity of your glaze slurry. One reason for requiring both factors to be checked is because ingredients like Epsom Salt, Sodium Silicate and Darvan are soluble, and have a potency that diminishes after a couple of months.

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"If these materials are fussy, and are going to cause more work for me, why would I use them?" Good question! For starters, none of these ingredients have an affect on the finished result of glazes coming out of the kiln; their function is purely to improve the working Greenware properties. Let me go through a couple common scenarios where these ingredients really shine. We will start with the flocculants:

• The flocculant that is most common in commercial glazes, which typically are designed for brush application, is CMC Gum. Gum helps keep glazes in suspension, retards the drying speed and increases both the adhesion to bisqueware and durability of the unfired glaze. To use gum in a recipe, add it to a little water first, just like you would add corn starch to soup. Once you have a homogeneous gel you can mix in the remaining water.
  

 

• Epsom Salt is the go to flocculant in community and school studios mainly because it is cheap and can be added as needed to an already made bucket of glaze. In a situation where a glaze keeps settling hard in the bucket, it is likely because the clay percentage in the recipe is low and/or the water content is high. When you try reducing the water percentage to minimize settling, the glaze applies too thick, or it dries too quick. When a glaze dries too quick it can be hard to smooth out thickness variations as it becomes chalky and difficult to apply; you will see tong marks and drip lines from dipping, and brush strokes will cake on the surface, leaving a frustrated artist in its wake.
  

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Instead of adjusting the chemistry of the glaze formula or reducing the water content, try adding Epsom Salt into the bucket, stirring well between additions as it can take a minute to dissolve and take effect. In a five gallon bucket of glaze, you can expect to add around a tablespoon of Epsom Salt to see a notable thickening of its viscosity. Its strong stuff, so add a little at a time so you don't overdo it!

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When it comes to deflocculants like Darvan 7, Darvan 811, and Sodium Silicate it is imperative that you THOROUGHLY and AGGRESSIVELY mix your slurry prior to use. Deflocculants activate with agitation, so improper mixing can lead to you adding too much. Once you have added deflocculant to your slurry you can identify the correct amount of mixing by mapping the deflocculation curve. To do this, mix with a drill, stopping every minute to check the viscosity. The longer you mix, the lower the viscosity will drop; once the viscosity stops dropping you have mixed it enough. Okay, that was pretty technical, thanks for sticking with me. Now for a couple tried and true scenarios where you will benefit from using deflocculants:

• Casting slip without deflocculant is nearly impossible to use. In order to get a workable consistency with water alone your castings will water log your plaster molds, shrink too much and crack. Just make sure you aggressively mix your casting slip to activate your deflocculant. If you are making casting slip from scratch, add the deflocculant to the water prior to adding the dry ingredients to save yourself a huge headache.

 

• Decorative engobes for dip or pour application greatly benefit from adding deflocculant. Since these slips get applied to leatherhard or chocolate bar stiff work, keeping the water percentage low will keep your pot from over-saturating with water and collapsing.

 

• Some glazes require a ridiculous amount of water to flow properly. Shino, I'm looking at you! With water alone, a high water content glaze will water log your pottery, taking forever to dry before you can handle it. In some scenarios the pot will be so water logged that the glaze won't adhere to the bisqueware and flakes off. Subbing out some of that water with Darvan 7 can eliminate this frustration.

 

• When it comes to the spray application of glaze you can only work on one pot at a time in the spray booth, so being able to spray quickly and handle the piece immediately can be a huge time saver, especially for those of you working as production potters.  Building up multiple coats of glaze that dries on contact requires getting the water content as low as 45-50%. To get that low water content glaze to flow through your spray gun requires an ideal flow rate through a Ford #4 cup of 10-12 seconds.
  

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In my studio practice I frequently make up multiple batches of the same glaze, each optimized for application method, depending on if I need to brush, spray, or dip. Batches can differ by up to 40 percent water, then using flocculants and deflocculants I bring those varying batches to the same viscosity. There are a lot of great ways to improve your experience in the studio. With a dash of this or a pinch of that it will get easier to see your artistic vision become reality.

I hope you found this article helpful. -Elliott