The antimension can only be used by Papal indult. But perhaps it will be given freely or perhaps an altar-stone could be used .
Surprisingly, the rubrics of the missal do not directly mention the relics in the altar. They only make reference to the altar being validly consecrated. I suppose that supposed the relics in the altar especially as mentioned in the Pontifical and laid down in the 1917 Code of Canon Law and decrees of the Congregation of Rites.
Digressing slightly, as regards validly consecrated altars, I’ve copied part of an article written due to the fact that Ireland had, until the beginning of the 20th century, a faculty for celebrating without relics.
St. Liguori tells us that there are two opinions of theologians on this question. Some theologians thought that relics were necessary for the consecration of an altar. This opinion was founded on certain texts of Canon Law, and on the custom of the Church, which according to these theologians was sufficiently manifested by the prayer in the Missal, " Oramus te Domine per merita sanctorum tuorum quorum reliquiae hic sunt," &c.
The second, and more common opinion, on the contrary, denied that relics were necessary for the consecration of altars. For this opinion St. Liguori quotes such names as Suarez, Lugo, Laymann, Vasquez, Palaus. These eminent theologians argued that there was no clear precept commanding the insertion of relics in altars ; or, if the precept ever existed, that it had been abrogated by contrary custom. And as to the prayer in the Missal they replied that it should be understood conditionally, " Ad orationem autem illam Missae Quorum reliquae, &c., respondent intelligi sub conditione si adsint."
When, therefore, these distinguished theologians tell us, that if there ever existed before their time a precept requiring the insertion of relics in altars, it had been abrogated by contrary custom, we may infer that Ireland has not been singular and exceptional in celebrating for a time on altars unhallowed by martyrs’ relics.
**Does an altar lose its consecration by the loss of its relics? **
At present we only consider what theologians taught on this subject. In our next question we shall draw attention to the decrees of the Sacred Congregation bearing on this
Well, again, St. Liguori tells us there are two opinions. Some theologians, and among them Suarez, held that altars do not lose their consecration, if the little slab which closes the sepulchre of the relics be broken; nor if the relics themselves be removed. Because relics are not necessary for the consecration of an altar in the first instance ; and, therefore, a consecrated altar does not lose its consecration by their removal.
Theologians, however, more commonly held that an altar loses its consecration when the little slab enclosing the relics is broken, or when the sepulchre with its relics is removed. They argued principally from custom, " Eatio quia talis est consuetudo Ecclesiarum ut altaria, fracto sigillo vel amoto sepulchre iterum consecrentur."
The reason given by Laymann for this opinion is interesting in view of certain decrees of the Sacred Congregation of Rites to which we shall have to refer. Laymann taught that relics are not necessary for the consecration of an altar. And yet though he admitted the probability of the opposite opinion he held that an altar should be reconsecrated if relics had been placed in it at its first consecration, and subsequently lost, by the fracture of the slab which closes the sepulchre, or by the removal of the sepulchre. He argues that the altar loses its consecration not exactly by the loss of the relics, but by the substantial fracture of the altar which necessarily takes place. He writes, “Nihilominus puto servandam esse Ecclesiarum consuetudinem, communi Doctorum sententia firmatam, ut altaria iterum consecrentur; cum enim sigillum hoc [the slab which closes the sepulchre] censeatur praecipua pars mensae, merito ob ejus fractionem, vel amotionem ipsa mensa sive lapis notabiliter confractus vel diminutus aestimatur."
Considering the diversity of theological opinion on this question, which is somewhat akin to the preceding, we again conclude that it is no wonder if even yet there maybe found in Ireland some altars consecrated without relics.