Adding noise/speckles to slip

[Mmplin]

Hello! I am trying to figure out a way to add small speckles, almost like TV static, to my unglazed slipcastings. Here are a couple images I found that show the effect I'm trying to learn: 

 

Thanks so much for any help!

[Min]

Are you making your own slip or using commercial slip? 

[Mmplin]

I've been using a commercial slip but will be adding mason stain so it's colored

[Min]

If you don't have the option to purchase a speckled slip then I would try adding up to 0.25% (that's zero point two five) of 60-80 mesh granular manganese. 

edit: I would guesstimate the amount of water in the casting slip as 35% of the weight of the slip and run some tests with up to 0.25% of the granular manganese based on that weight.