Corgi, that is a really nice mug. I am not sure how it was originally used but it sounds like you are having a lot of fun trying some new lathering techniques. I just purchased a neat old mug myself and cannot wait for it to arrive. It is smaller but I believe it will be perfect for building a lather with cream. There will be more bristles focused on the product and more contact in the tiny space so less time swirling.
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Thanks to this thread I am going to soak the milk jar in hot water first. I will be ecstatic if I can get warm lather in such a simple fashion!