I don't think I have to translate "liquidus" and "solidus", even if it is Latin.
First, we have to understand binary eutectic alloys (see picture 1):
We have a liquidus-line. Above that, everything is liquid.
We have a solidus-line. Below that, everything is solid.
So area (1) is completely liquid.
Area (3) is completely solid.
At the eutectic point, the alloy changes completely from solid to liquid.
Now what's on in the areas (2)?
Here, we have a mix of solids and liquids. In a binary system (components A and B), the further we are away from the eutecticum, the more solids of component A or B we have "swimming" in the liquid. This is the superfluous component to get a perfect eutectic alloy.
Now, if we look at picture two:
This is a oversimplified Fe-C diagram. It is only useful right of the 4%-point. The important point is, that it doesn't stop at 10% C, it goes all the way up to 100% C (quite useless from the POV that we want a cast-able iron).
But if you accept that the area (2) of the first picture contains a mix of solids and liquids, we see that at 10% carbon content, 6% are solid Fe3C. These Fe3C are small crystals, no big blobs or whatever that considerably change the viscosity.
Nick