It is a fact that may not appear obvious but the use of silk or even cotton is not approved for the liturgy.
Instead the Church has always prescribed materials made out of 100% linen.
There is a lot of linen used in the liturgy: Altar cloths, corporal, amice, alb, pall, lavabo towel, purificator, etc. The origin of this practice around liturgical fabrics goes back to the Old Testament. The book of Leviticus prescribed that the priest to be dressed in a linen tunicle for the sacrifice: “He shall be vested with linen tunick. He shall cover his nakedness with linen breeches. He shall be girded with a linen girdle.” (Lev. 16:4) Other references can be found in Exod. 28:39, Exod. 39:27, 1 Para 15:27 and Ez. 44:18.
“High-quality linens are soft, smooth and have no impurities.”
Linen was used as an image for purity. Not only does it’s white color indicate cleanliness and purity, it is also a precious fabric. Manufacturing linen is laborious process which makes it costly. The end result are high-quality linens that are soft, smooth and have no impurities.
By using linen, the Church also references to the fact that our Lord’s body was wrapped in fine linen on Calvary. “And Joseph buying fine linen, and taking him down, wrapped him up in the fine linen, and laid him in a sepulchre which was hewed out of a rock.” (Mk 15:46) That linen cloth referenced in the Gospel is commonly known today as the Shroud of Turin.
“In the Old Testament, mixed fabrics were forbidden to priests.”
Mixed fabrics forbidden
In the Old Testament, mixed fabrics were forbidden to priests in “Thou shalt not wear a garment that is woven of woolen and linen together” (Deut. 22:11) and “Thou shalt not wear a garment that is woven of two sorts” (Lev. 19:19).
Josephus suggested that the reason for the prohibition was to keep the laity from wearing the official garb of the priests, while Maimonides thought that the reason was because heathen priests wore such mixed garments
Why are cassocks then not made out linen? Very simply because the cassock is not part of the liturgy. It is the daily garment of priests and the Church does not require it to be precious. It suffices that the priest or sacred ministers cover themselves in linen during the liturgy itself.
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