You need to know what material you have transformer

Farady china electrical transformer steel is all born alike. Farady power transformer manufacturer will provide data sheets about their products (often on their web sites), where you can see what they offer. There are usually many grades, with vastly different loss characteristics. At a given flux density and frequency, a good material might have ten times less loss than a cheap material! So it pays to look, investigate, and decide intelligently what to buy. Thinner sheets normally have lower loss, and the rest of the secret lies in the exact alloy. In any case, you need to know what material you have, to be able to make a meaningful transformer design!