I am assuming that you are talking about a pipe or porous semi hollow
inside the ingot, not on the ingot surface although you might see
some porosity there also.
If so you are having a problem with dissolved oxygen in the melt.
Molten silver can dissolve tremendous quantities of oxygen which
come out of solution on solidification.
If so you will need to correct your melt and pouring practices. This
may not be easy to do.
The melting temperature should be kept low and you must not "stew"
the melt. Melting should be done in a vacuum or inert or reducing
atmosphere. You can help by floating carbon over the melt to keep
oxygen out. You have to be careful not to add oxygen during the pour
and not carry carbon bits into the mold.
The mold should be hot and and you must control the cooling so it
occurs from the bottom upward with the sprue the very last to cool
and solidify. An insulating cap will help will help keep the cooling
somewhat controlled. This last portion will probably be porous -
hopefully you can cull this. It used to be a practice to deoxidize
with a bit of cadmium but this produces dangerous fumes.