der Klumpen
lump
An irregularly shaped, solid mass made of a particular material.
Sie formte aus dem Teig einen großen Klumpen.
She formed the dough into a large lump.

lump
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lump
An irregularly shaped, solid mass made of a particular material.
Sie formte aus dem Teig einen großen Klumpen.
She formed the dough into a large lump.

The related verb 'verklumpen' means that a substance (e.g., flour in water) forms solid, irregular pieces or clumps. This helps you understand the basic meaning of 'Klumpen' (lump) as a shaped mass.
Das Saucenpulver ist im heißen Wasser verklumpt.
The sauce powder clumped in the hot water.
In its neutral meaning, 'Klumpen' (lump) is often combined with the material the mass consists of. The typical structure is 'ein Klumpen [material]' (a lump of [material]), for example 'ein Klumpen Lehm' (a lump of clay), 'ein Klumpen Gold' (a lump of gold), or 'ein Klumpen Eis' (a lump of ice).
Im Garten fand er einen großen Klumpen Lehm.
He found a large lump of clay in the garden.
This meaning is colloquial and always negative (pejorative). It expresses that a thing or person is perceived as shapeless, heavy, old, or clumsy.
Was für ein alter Klumpen von einem Auto!
What an old hulk of a car!
You can recognize the colloquial meaning when 'Klumpen' (lump/hulk) refers to a finished object (e.g., a piece of furniture) or a person and the tone of the sentence is derogatory. The neutral meaning usually refers to raw materials.
Beweg dich mal, du alter Klumpen!
Move it, you old lump!
mass / crowd
'die Masse' and 'der Klumpen' can both describe something without a nice or precise shape. 'die Masse' refers more to a shapeless substance that may be soft, sticky, or mixed. 'der Klumpen', on the other hand, is a single piece of solid material, often irregularly shaped. They can be confused when talking about dough, earth, or other material. If you mean the substance as a whole, 'Masse' fits. If you mean a specific piece, 'Klumpen' is better.

Im Teig war ein harter Klumpen Mehl.
There was a hard lump of flour in the dough.
Die Masse für den Kuchen ist noch zu flüssig.
The mixture for the cake is still too runny.