One very good way to get the exposed parts of the foot and any chipped areas of porcelain clean is to use an ultrasonic cleaner. Ultrasonic cleaners use soundwaves in the ultrasonic range to vibrate the piece which loosens dirt that usually can’t be dislodged. I have used them to clean both underglaze decorated porcelain and overglaze enameled porcelain with good effect. I would warn against using ultrasonic cleaners with overglaze enamel that is in poor condition as it is possible to vibrate the enamel off the piece. I have never had enamel that is fused well to the glaze come off, and I have cleaned hundreds of pieces, but I guess it could happen.
For the pieces in the photos I used warm to hot water with a mixture of a little dishwashing soap, about 2-3 ml. of laundry bleach and about 1 tsp. of baking soda. Everyone has their warnings, cautions, and favorites when it comes to what to put into an ultrasonic bath. I have used many different mixtures and this is the one I find works very well to clean the exposed parts of porcelain. I would not recommend this much bleach for cleaning enamel. I use a very small amount, about 1 ml., when cleaning overglaze enamel. In overglaze enamel the mixture in the bath is in direct contact with the enamel and if there is too much bleach it might lead to fading. I have always used about 1 ml. with no ill effect. In cleaning underglaze decorated work the glaze acts as a barrier to direct contact with the decoration so a little more bleach can be used.
The photos show a blue and white plate that was run through a standard 5 minute cycle with no other cleaning. The 5 minutes in the ultrasonic cleaner removed all the ground in dirt from a piece that is about 160 years old.
In the photos the lid with the rust mark was run through 2 cycles of 5 minutes each and then I put it in a separate container with a stronger mixture of bleach/baking soda for about 15 minutes.
Some tips on using these types of cleaners.
When running the cleaner, especially with very thin porcelain, try to not let the piece lean on the edge of the cleaner. The machine is actually sending out sound waves that act to vibrate the item being cleaned. This vibration also causes the machine to vibrate. The piece being vibrated along with the bath vibrating can lead to cracks if the piece is leaning on the side of the bath. The situation wherein the piece is leaning on the side of the bath is usually because the piece is much larger than the bath. This leads to my second point.
Buy a cleaner that is large enough for what you want to clean. If you are a collector of large porcelain chargers you may not be able to find a large enough cleaner but if your collection consists mainly of smaller plates and cups you should be able to find a cleaner on the Internet for about 20-40 dollars.