The Silver Commemorative Coins of Finland

I have been trying to add silver to my collection and have focused on the modern commemorative coins of Finland, I like their eclectic designs. I have noticed something unusual in the the ASW of silver in these coins varies from year to year. Example the the 10 Markkaa coin of 1970 has an ASW of .3657, the 1971 is .3890 and the 1975 is .3778. Any idea of why the variance? In my past experience, which I admit is not that great, the ASW of a specific denomination usually remains the same from year to year. An example being all the Classic Commemorative Halves of the United States have an ASW of .3617 from 1892 to 1954.

Weight of those coins is different and that's why ASW is different. Coins are .500 silver.

I could imagine the mint had a certain budget and the amount relates to the current silver prices at the time?

Verweis : "Idolenz"​I could imagine the mint had a certain budget and the amount relates to the current silver prices at the time?

​Doesn't seem to correlate. Below are year, average closing price silver (USD), coin weight (g), mintage:

1970 1.77 22.75 600 000
1971 1.54 24.2 1 000 000
1975 4.43 23.5 1 000 000

OK, now I got it. 10 markkaa 1971 has same size and silver content but that coin has 5 % of zinc, which cause the ASW difference to 1970 coin. Zinc was used because the metal was obtained by smelting old zinc-containing silver marks. Same mixture were used in 1975. So 10 markkaa 1970 is Ag 50 %, Cu 50 %, weight 22,75 g, 10 markkaa 1971 is Ag 50 %, Cu 45 %, Zn 5 %, weight 24,2 g and 10 markkaa 1975 is Ag 50 %, Cu 45 %, Zn 5 %, weight 23,5 g. After these coins the content is 50/50 % again without zinc.

Verweis : "Ollisaarinen"​OK, now I got it. 10 markkaa 1971 has same size and silver content but that coin has 5 % of zinc, which cause the ASW difference to 1970 coin. Zinc was used because the metal was obtained by smelting old zinc-containing silver marks. Same mixture were used in 1975. So 10 markkaa 1970 is Ag 50 %, Cu 50 %, weight 22,75 g, 10 markkaa 1971 is Ag 50 %, Cu 45 %, Zn 5 %, weight 24,2 g and 10 markkaa 1975 is Ag 50 %, Cu 45 %, Zn 5 %, weight 23,5 g. After these coins the content is 50/50 % again without zinc.

​Except they don't have the same size. 1971 is significantly thicker and hence heavier. Replacing 5% of the copper with zinc would have actually lowered the weight if they were the same size.