Mulling

[helmarshausen]

Hello,
I just received an order of pigment from Sinopia. Is it possible to make the pigment paste without a muller, like by just using a palette knife. the pigments are ultramarine, yellow ochre, terre ercolano, titanium white, ivory black, verona green earth with viridian, english burnt sienna.

Also, someone on this forum mentioned just shaking the pigments with water in a baby food jar. Wouldn't that make the paste too fluid? or is that okay? How thick must the paste be?

JohnCline

[brianhendrickson]

Hi John,

I've been using Sinopia pigments since I started in egg tempera. The pigments I've gotten from them are ground finely enough that the muller seems unneccessary.

Normally, you'd just add a little distilled water to the pigment, then mush it around with a palette knife until you get the paste consistency that seems suitable. Then you add a bit of the paste to the egg medium (again, consistency is up to your own personal preference) and start painting.

I store the pigment paste (without the egg mixed in!) in baby food jars so I don't have to mix up a new batch for every painting session.

Hope this helps!

Brian

[Georgeoh]

The pigments I've gotten from them are ground finely enough that the muller seems unneccessary.
The purpose of "grinding" pigments with a muller on a slab is not to grind the pigment into finer particles, but to break up agglomerations of pigment particles into their discrete particle sizes and to better disperse the pigment in water (or other medium). It would take days, weeks, perhaps even months of continuous grinding to actually further breakdown the discrete particles using a muller. (A mortar and pestle is used to breakdown pigment particles.)

When you simply mix the pigment with water using a palette knife you do not disperse the pigment in water or medium, but allow agglomerations of particles to remain, and hence the recommendation to "grind" or mull pigments with a muller.

Pigment particles are typically hydrophobic, meaning that they resist mixing with water. The electrical charges on the surface of pigment particles resist water molecules from attaching to the surface of the particle, thereby allowing air to remain in these intercises. In addition, particles "stick" together because of the electrical charges forming agglomerations with air in between the particles. This is especially true for finely granular pigments as they have much more surface area than pigments with larger particle sizes.

[Georgeoh]

In dispersing dry pigment powders in a paint vehicle there are several steps that must be followed:

The wetting step consists of replacing the adsorbed materials on the surface of the pigments and inside the agglomerates (water, air, etc.) by water (or paint vehicle).

The complete wetting out of the primary-sized pigments particle helps to enhance the performance of paint that depends very much on the interaction between the pigment particles and the binder. Dispersing additives, which adsorb on the pigment surface, facilitate the liquid/solid interfacial interactions and help to replace the air/solid interface by a liquid medium/solid interface.

The efficiency of the wetting depends primarly on the comparative surface tension properties of the pigment and the vehicle, as well as the vicscosity of the resultant mix. The adsorption mechanism depends on the chemical nature of the pigment and the types of dispersing agents, if any, used.

The wetting step of dispersing processes can be intensified by the use of wetting agents and/or binders with lower viscosity and surface tension. On the other hand, resting the pigment/binder premixes prior to their dissolving or grinding helps to accomplish wetting and always eases and accelerates dispersing processes.

After the wetting the pigment, it is necessary to de-aggregate and deagglomerate the pigment particles. This is usually accomplished by mechanical action provided by mill equipment in industrial settings and in the studio with a muller and slab.

In the grinding stage, the cohesive forces inside the agglomerates have to be overcome. Mechanical energy is added to the system and therefore smaller particles (with a larger interface) are formed. This results in loosened inter-particle contact, which eases the destruction of pigment clusters under the action of shear stresses applied by the muller.

As the pigment powder is broken down to individual particles by mechanical shear, higher surface areas become exposed to the vehicle and dispersing agents may be required to wet out newly formed surfaces.

Once dispersed, the primary particles have a tendency to re-agglomerate. This process is called flocculation. From a structural standpoint, the flocculates are very similar to the agglomerates; nevertheless, the interstitial spaces between the pigments are now filled with water (or paint vehicle) rather than air.

The grinding process can be regarded as a de-flocculation process. In the absence of stabilizing agents, effects such as reduced color strength, decreased gloss, and altered rheology then may occur.