Actually "antiplastic" is OK, and "aplastic", as a noun or adjective, is quite commonly used, but "temper" is the classic pottery term.

Although "desengrasante" is a synonym for "antiplástico", as Marie says, it doesn't refer to grease. It's because very plastic (pliable) clays are referred to as "grasas" in Spanish. An antiplástico is a substance added to the clay to reduce plasticity, which, as your author says, can lead to firing loss. Here's part of a good though somewhat lengthy definition:

"Antiplástico
Algunas materias primas utilizadas en la composición de pastas cerámicas se denominan antiplásticas, en oposición a las materias primas plásticas, porque disminuyen la plasticidad de las pastas. También se suelen llamar desengrasantes porque a las arcillas muy plásticas, debido a una untuosidad especial que tienen al tacto, se les califica como grasas, y por adición de un desengrasante disminuye esta característica. Los materiales antiplásticos se utilizan para mejorar el comportamiento en el secado y en la cocción de las pastas, disminuyendo su contracción y deformación. El desengrasante más utilizado es el cuarzo, que aparece en la composición de muchas pastas cerámicas. Al componer pastas cerámicas, un punto de partida normal es utilizar cuatro componentes, dos plásticos y dos antiplásticos. [...]
Otro tipo de material antiplástico utilizado en la composición de pastas cerámicas es la chamota. [...]"
http://ceramica.name/tecnologia_ceramica/Diccionario/Diccion...

Chamota, by the way, is grog: ground fired clay.

Well, "antiplastic" is used in technical literature, and not only in translated texts:

"Added antiplastic is grog in five out of the seven samples."
Donald B. Redford, Excavations at Mendes: The Royal Necropolis, 214
https://books.google.es/books?id=brHXALL4pRsC&pg=PA214&lpg=P...

"Aplastic" is more common:

"aplastics
SYNONYMS OR RELATED TERMS: temper
CATEGORY: ceramics
DEFINITION: Intentional or accidental inclusions in pottery clays before firing."
http://www.archaeologywordsmith.com/lookup.php?terms=temper

"Clays used to manufacture the Great Gully Farm ceramics contained many more natural aplastic inclusions than with the Corey ceramics"
https://books.google.es/books?id=iYH3CgAAQBAJ&pg=PA73&lpg=PA...

But "temper" is most common for this:

"temper
SYNONYMS OR RELATED TERMS: grog
CATEGORY: ceramics
DEFINITION: Foreign material (sand, plant fibers, grit, shell, crushed rock, broken pottery) added to clay for potterymaking to improve its firing qualities and prevent a vessel from cracking during the drying process. Temper reduces plasticity, which would cause shrinkage or cracking upon drying and firing. The study of temper is important for the identification of the place of manufacture of a vessel."
http://www.archaeologywordsmith.com/lookup.php?terms=temper

http://practicalprimitive.com/tempering.html

"What is Temper?
• “substance in clay that modifies its properties when wet or dry as well as during and after firing” (Rice 1987:407)
• “mixing of additives (including other clays) with clay to produce a body for pottery making” (Rye 1976:109)
• “nonplastic inclusions” (Bronitsky and Hamer 1986:89)
• “nonclay inclusion in paste--either added intentionally by the potter or present naturally” (Skibo et al. 1989:123)

What Does Temper Do?
• Actually weakens the ceramic potential of clay, but…. it is needed to:
– improve workability of clay (decrease or increase plasticity)
– reduce shrinkage and cracking during drying
– manipulate firing behavior to achieve desired effects (e.g., act as flux)
– manipulate porosity
– manipulate bulk density
– manipulate thermal properties (e.g., insulation, conductivity, shock resistance)"
http://users.clas.ufl.edu/sassaman/pages/classes/ceramics/Po...