Flocculation
Flocculation is the process of adding material to a glaze or any solution with clay particles as part of its makeup, to increase suspension properties. The idea is to keep a liquid that is no longer in motion (such as by stirring, vibrating or shaking) from settling by adding a material that causes the clay particles to attract one another and clump up in the solution. This can be looked at in several ways.
First one has to have at least a small amount of clay in the mix for a flocculant to work. It is the clay particles in their tiny platelet form that have the magnetic charges that are affected by the degree of flocculation. So, when looking at a glaze, one needs to know if one of the ingredients is clay. If no clay is found in the recipe, the clay content can be added in the form of bentonite at 2-4% which shouldn’t change the fired outcome of the glaze too much. I use additional bentonite in all my glazes, but if the clay content in a recipe is over 20% then no bentonite is needed for this process.
When do you need to flocculate a glaze? I do it to all glazes when first mixing them up as a precaution, but as the glaze ages in its bucket it becomes apparent which glazes need additional flocculation. When no flocculant is added then time shows that some glazes will form a hard layer on the bottom of the bucket which can only be remixed by scraping and cutting with a metal object. I use a trimming tool or carving tool when this happens to pry up the hardened layers. But if the glaze had been flocculated in the first place then this annoying chore would not need to take place.
Above, I suggested that flocculation can be looked at in several ways. Simply put, the process of flocculation is slowing the solution from settling or placing the solution into suspension. Or you can think about it as turning the solution into a gel. But how is this done?
Clay particles are flat platelets that have positive and negative charges that both attract and repel. When these charges only attract one another the liquid is said to be flocculated. These flocculated groups of clay particles are loose and irregular with space in between them as the clay platelets are not neatly stacked together like bricks in a wall, but are sometimes perpendicular to each other. Flocculation not only keeps the clay particles clumped together in irregular shapes, but stops the larger particles that are sometimes hundreds of times bigger than the flocculated clay particle groups, from sinking to the bottom of the bucket.
A flocculated group of clay particles is acidic in nature. So to change a solution with clay materials one must increase the acidity of the solution, thus changing the polarity of the materials charge. So, another way of looking at flocculation is changing a glaze from a high PH to a low PH by adding acidic solutions. The additives must be non oxides or they will change the outcome of the glaze effects and color, so specific materials are used.
The usual substances added to glazes with clay particles in them in order to flocculate the glaze are as follows:
Epsom Salts (hydrous magnesium sulfate) Vinegar (dilute aretic acid) Calcium Chloride Plaster (hydrous calcium sulfate) Flocs Snow White (anhydrous calcium sulfate) *Muriatic Acid (dilute hydrochloric acid) Aluminum Chlorate
*Always add acid to water. Never the other way around as the combination of acid and water causes an exothermic reaction, a chemical reaction creating heat, boiling the water which could splash acid on the individual.
Epsom Salts is the main flocculant added to glazes. I put a pinch in when mixing up a glaze, but a solution can be made by taking hot water and then adding the maximum amount of Epsom Salts to create a saturate solution. This solution can then be slowly added (by no more than a teaspoon) until the desired viscosity has been achieved. Whatever method is used must have the flocculant added by very small increments as to not over flocculate the glaze. Over time more flocculant may be needed and as before, slowly add more until the desired affect results.
When a glaze is flocculated it will allow one to dip into the glaze to achieve an even coat as well as allow one to pour a glaze onto or into a porous material to create an even coat, but there are times when one would not want a flocculated glaze. Usually, this is when one has a gloss surface or nonporous surface that one wants to coat with glaze. If a flocculated glaze is placed on a nonporous surface it will take a long time to dry and may not adhere in the firing. In this instance one would want to apply a deflocculated glaze.
Deflocculation
Deflocculation is used when a glaze or slip is needed to be suspended with a minimum amount of water. By creating a deflocculated glaze or slip with little water one is able to create a thick solution that flows well, but dries quickly. Slip casting clay bodies are usually deflocculated so they can be poured into plaster molds in an even consistency that dries quickly allowing the remainder to be poured out of the mold. Also, underglazes and slips are deflocculated so they brush easily, adhere to the desired surface and dry quickly. A properly deflocculated glaze or slip will not allow the paint brush to stick to the clay surface when attempting application.
When mixing a solution with little water a thick mud is created that is difficult to move through. By adding a deflocculant it is as if the solution has been made frictionless allowing one to easily mix with minimal water. In fact, what has happened is the electric charges of the clay particles have been rendered neutral by adding an alkaline or basic material to the mix reducing viscosity.
The viscosity of a solution can be measured with a viscometer and is called the solutions specific gravity or this measurement can be determined mathematically when liquid and dry material are mixed, though temperature, humidity and chemicals in the water supplied can quickly alter the specific gravity of a solution.
Deflocculation occurs naturally in glazes that have high percentages of soluble basic materials such as wood ash, soda ash, nepheline syenite, some frits and lithium compounds. The soluble materials leach out into the glaze and cause settling. Shino glazes and those with high amounts of frits and low amounts of clay are usual culprits, other examples being oxblood or copper red glazes. One must counteract this with acidic material additions as explained above under “Flocculation”.
There are times when Deflocculation is purposely desired and it is achieved by adding basic materials to the slip or glaze which include thickeners, binders and additives that help flow and brushability. In essence one is making paint. If you look up any companies paint recipes (if they allow you to) you will see fillers and opacifiers, dispersing agents, thixotropes or plasticizers, gums and thickeners (binders), preservatives and conditioners. These are all items that we can put into a glaze or slip to create a consistency one desires for the application at hand.
Sodium Silicate which is the main deflocculant used for slip casting clay bodies, slips and underglazes must be used in small allocations as too much of this material can have the reverse effect causing the solution to turn to pudding. The additive will case the solution to get thinner to a point and then when too much is added the solution will become thicker. This amount is difficult to determine.
In order to use any deflocculant, first one must ensure clay is present for the deflocculants to work and if none are present then bentonite is added (thixotropes). By adding our deflocculant (dispersing agent) to the glaze (fillers, opacifiers) we are still left with a solution that may not be very brushable or usable. At this point the solution will settle slowly as the neutral particles are jumbled together with the little amount of water in between them allowing flow and creating suspension. The next additives will also help with suspension, but the main trick is to blend all the materials together to get them to gel or stay in suspension. Vigorous mixing is essential.
Gums and Thickeners are added to make the glaze or slip stick to the application surface. This is done by thickening the solution with CMC Gum, Gum Arabic or Gum tragacanth. These gums are notorious for not mixing well in water, so first mix up a saturate solution of the gum and hot water and then calculate how much is added to the glaze or slip solution. Additional thickeners that are added can be propylene glycol and glycerin. I add a gum and then both the propylene glycol and glycerin. The later add significantly to the brushability of the glaze or slip as well as slight deflocculation qualities.
I don’t add preservatives, but some of the materials listed are organic and can rot and smell, so preservatives are a viable option. The gums are organic, but I’ve never noticed a problem with them smelling. Also, I’ve never added a conditioner, but they help stop foaming if that is ever noticed. I think with enough thickeners no conditioners are needed in slips, but may be needed in high alkaline glazes which may be deflocculated to brush on nonporous surfaces.
As noted earlier the amount of basic materials or compounds in a glaze varies, so it is difficult to know how much of each material to add. First, the amount of clay in the glaze will determine how much of the thixotropes or plasticizers are needed. Then the basic nature of the glaze or slip must be ascertained to determine the amount of deflocculant to add. Then once this is determined, the amount of thickeners and gums and flow agents can be added in small quantities. Even then, it may take several renditions of the same glaze or slip to determine the correct quantities.
Deflocculants are as strong if not stronger than their flocculant antithesis, so use very little to start with and then additions as needed. a small amount can easily turn the glaze or slip into pudding, so be careful.
Common Deflocculants:
Dodium Silicate Darvan 7 (long chain sodium polyelectrolyte) Darban 811 (short chain sodium polyelectrolyte) Soda Ash (Sodium Carbonate) Sodium Tri-polyphosphate Tri-sodium Phosphate Tretrasodium Pyrophosphate Additive-A (Types 1 to 4) (Calcium Lignosulfanate) Additive-A 373 (a sodium lignosulfonate co-polymer) Darvan 404 (Calcium lignosulfonate) HSD 214 Amonia Citrate
Thixotropes or Plasticizers
Bentonite (white and grey) Veegum T or Macaloid “Bentone” (colloidal magnesium aluminum silicate) Veegum Pro (amine treating veegum)
Gums, Thickeners and Binders
Bentonite Veegum T Veegum Pro Veegum CER (combination of Veegum and CMC) Macaloid CMC Gum (Sodium carboxymethylcellulose) Methylcellulose Microcrystalline Cellulose Gum Arabic (Regular and Glass painters grade) Gum Tragacanth Dextrine (starch) Sugar Syrup Gelatin
Thickeners and Flow Agents
Glycerin ( C3H5(OH)3 ) Ethylene Glycol (poisonous auto antifreeze) Propylene Glycol (“safe” antifreeze, Sierra) Liquid Laundry Starch Floetrol (Flood Company paint additive)
I want to mix a color slip to apply to wet clay. A suggestion to me was to use the clay body I am throwing with. . I use B CLAY which is similar to B CLAY . Do I need to add a deflocculant ? and what deflocculant would be good to use? Is bentonite considered a deflocculant?
I meant to say B CLAY Is similar TO B MIX.
No problem and sorry for the late reply. You shouldn’t need to use a deflocculant, but you may need to add some flux if you are adding a large quantity of mason stain to your slip as mason stains are refractory. Try the clay body slip out first with no additives on a test piece and if it is too dry, cracking or flakes off a little, then add some flux. If it works, you don’t need to add anything. If you are brushing it on your clay body, then you can add one of the flowing agents listed in the article. You won’t need much. good luck!
Hi, very interesting article, thank you. Is it possible to add a deflocculants to the glaze, then at a later stage add flocculants to thicken it? In other words, can they both be present or does one cancel the other out?
Thanks Arthur, Great question. There can be both deflocculants and flocculants in a glaze at the same time. You can add deflocculants to a flocculated glaze and vice versa, but they don’t cancel each other out in a straight forward way. There are so many factors that it gets complicated fast. The best way to figure it out is to test and go very slow (as once you turn the glaze to pudding it is difficult to go back), but I’d say it is safe to go back and forth once, but after that, you may have problems as you are dispersing (deflocculating) and coagulating (flocculating) molecules essentially by messing with the number of electrons they have. And as I am not a scientist, I’m not sure how many times you can change the PH of a solution based on the amount of clay (which is what you are deflocculating/flocculating) and the amount of additives you put in. I Hope that helps!
Thank you for your reply. It’s really generous.
Hiya, I am trying to produce a slip for raised trailing but to be used in bisque tiles. My base is an earthenware casting slip (SG 1.70 ish). As i am using bisque rather than green-ware im concerned about line shrinkage.
Im trying for a prominent raised effect for the lines. As the casting slip is already defloculated do i just add clay powder or would i need a little more Sodium Silicate as well to the correct viscosity.
Im just starting to experiment with small volumes but any advice is much appreciated.
Hello Stuart, Did you make the tiles out of the casting slip or are they store bought? I would take a tile either way and mark a line of casting slip as is, then one with a little clay then maybe 10% more clay up to 30% clay additions. This should give you an idea of how clay additions will help, then try adding a little flux, then 10% flux (whatever flux is in the slip recipe) up to 30%. This should give you results on whether the slip is too refractory. If you don’t know the correct flux in the slip try frit 3124 or similar frit for low temps. I wouldn’t add any more sodium silicate as the slip is already deflocculated. Keep in mind if you add colorants (mason stains or oxides) you will have to add additional flux to counter balance the refractory nature of the colorants. Though, some oxides are a flux (like iron) so you won’t need any additional flux if you are firing to a temp. where the oxide will melt (iron at low temps. may need flux as it melts at high temps.). Hope this helps.
What can you do if you have ended up with “pudding”??? You mention that it is hard to go back once pudding happens… what can you try?
Thanks
Hello Sarah, “pudding” usually means you have put too much in, so the only way to reverse it is to add more of the base to reduce the amount of deflocculant. It is possible to add a flocculant, but it is very difficult to judge the amount to add.
Hi, I am working in ball clay producing testing. we do many physical and chemical testing of clay. i generally found some clay does not defloculate, and some of them defloculated very easily. is there some relations of defloculant demnd for a good flow. or some parameter that affect pure ball clay defloculant demand.
Hello Piyush, I’m not sure if I understand your question, but hopefully this will help. You need to have a sufficient amount of clay in the slip for it to deflocculate. There are other materials that will work, but clay is the best. It may be that the particle size of some of the ball clay tests are too large or that they are all the same size, so they don’t deflocculate as well as others.