Does butter get rid of the scum? I am new to this jam making 
Yes using a little knob of butter will stop the scum. I think scum is a horrid word for it tho' as it is trapped air bubbles.
You can also use a teaspoon of sunflower oil instead of the butter as some folk are allergic to dairy.
I think I have cracked it with strawberry jam now.
I use 4lb strawberries, 8oz of grated bramley apple cooked in enough water to stop it from burning.
and 4lb of sugar and 5 tablespoons of lemon juice..
Once the apple is mushy I put it with the lemon juice and strawberries and cook gently until the strawberries are soft and the pectin is extracted. If I think it is ready I do the methylated spirits test to check and then add the sugar and boil to a set.
I don't lilke using certo or jam sugar as they often have sulphites in, and the pectin powder doesn't do it for me at all.
You don't really want a stiff solid set as it is much nicer with a little movement when you put it over the scones and cream.
I have just made strawberry and passion fruit, strawberry and cointreau, strawberry and Kirsch, strawberry and vanilla, strawberry and elderflower and plain strawberry.
