Sometimes you may find that some of the Sil-Pad® Tube Series Round Thermal Pads from Bergquist have white powder inside the actual part.
For example, the part number SPT400-12-13.5-25 :
For reference, check this photo:
Bergquist confirms that the white powder inside the parts is talc only. The talc is used to aid separation.
Applicable part numbers
BER157-ND
211-2188863-ND
SPT400-12-13.5-25
2188863