Crystal image #004 - this image is actually precipitation of a poly-ethylene glycol (PEG).
PEG itself is a precipitant, which facilitates crystal formation.
The concept behind crystallization can be visualized with a solubility phase diagram (Image below).
image source: https://neutrons.ornl.gov/crystallization-solventexchange
The protein and precipitant are separated within a closed environment. The high concentration of precipitant in the reservoir (blue) causes water in the crystal droplet (red) to passively diffuse to the reservoir (image below), and this causes the concentration of the protein to increase over time. Ideally into the nucleation zone of the phase diagram. Simultaneously, the protein concentration decreases as the crystals begin to grow.
images source: http://soft-matter.seas.harvard.edu/index.php/Vapor_Diffusion_Method
please feel free to ask questions.
-The Scientist