This article is an excerpt from our Sefer
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A Shofar with engravings and drawings: 
One is not to draw pictures on a Shofar [even if done] in order to beautify the Shofar. However it is permitted to engrave drawings within the body of Shofar in order to beautify it. This allowance applies even if the engraving will alter its sound. [Nevertheless the custom is to avoid engraving anything on a Shofar.]
May one place a sticker with his name on his Shofar?
No. One is initially to avoid doing so.
Must one remove the manufacturer label from the Shofar?
Yes. One is initially to avoid blowing with it on the Shofar.
May one remove a sticker from his Shofar on Rosh Hashanah?
One may do so although not in the presence of an ignoramus.
 586/18; Michaber 586/17
 Those that draw pictures on a Shofar in order to beautify it are doing an improper action, even if the entire Shofar is not covered with drawings, as at times the sound of the Shofar changes due to the drawing. [Admur ibid; Tur in name of Ramban] This change of sound is caused by the ink which is similar to a coating. [Beis Yosef brought in Kaf Hachaim 586/122]
 Admur ibid; Rama ibid; Beis Yosef 586
 Admur ibid; Levush 586/119; Biur Hagra; Levushei Serud 586; M”E 586/26; M”B 586/73; Kaf Hachaim 586/122
The reason: As even if this causes a change in the sound, this change is Halachicly meaningless being that there is no additional material on the Shofar that is responsible for this change of sound and rather the change is coming fully from the Shofar itself, and all sounds of a Shofar are valid. [Admur ibid]
 Shulchan Gavoa 586/46; Kaf Hachaim 586/122
 Sheivet Hakehasi 4/160; Piskeiy Teshuvos 586/10
 See Admur 317/7; Our Sefer “The Laws of Shabbos” Volume 2 The laws of Cutting Halacha 1E!