PUTTING DOWN MUKTZEH THAT ONE IS ALREADY HOLDING [Muktzeh: putting down]
103a (Mishnah - R. Shimon ben Gamliel): Even one who bangs a hammer on an anvil [is liable].
Question: What is the reason?
Answer (Rabah and Rav Yosef): This is because he trains his hand [to hit with the proper force].
Objection (Benei Rachbah): If someone saw a craftsman on Shabbos and learned [from him], is he liable?!
104b (Mishnah): In the following cases he is exempt;
He wrote with [other] liquids, fruit juice, mud (alternatively - in dry earth), dust of residue left in an inkwell, or anything that does not last.
Rambam (Hilchos Shabbos 11:9): One who writes in order to ruin the leather is liable. He is not liable for the place of the writing, rather, for the writing. However, one who erases in order to ruin is exempt.
Magid Mishneh: This is from the Tosefta 12:7.
Rambam (15): One who writes is liable only for something that lasts, e.g. ink, red dye, sap, or vitriol, on something that lasts, e.g. leather, parchment, paper or wood. One who writes on something that does not make a lasting impression, e.g. liquids and fruit juice, or with ink on vegetable leaves or anything that does not last, is exempt. The same applies to erasing.
Rambam (16): One who writes on his flesh is liable, for this is skin. Even though his body heat will remove the writing after a while, this is like a [proper] writing that was erased.
Machatzis ha'Shekel (on Magen Avraham 340:7): The skin itself lasts, just something else, i.e. the heat, causes the writing not to last. This is like one who writes on leather. He is liable, even though over the course of time it fades.
Rambam (ibid.): However, one who scratches writing on his flesh is exempt. One who tears leather in the form of writing is liable. One who marks on leather in the form of writing is exempt. One who writes [black] ink over red ink is liable twice, for writing and erasing. One who writes black over black, or red over red, is exempt.
Rambam (17): Marking is a Toladah of writing. One who draws impressions and forms on a wall or lacquer, like artists do, is liable for writing. The same applies to one who erases an impression in order to fix. He is liable for a Toladah of erasing. One who scratches a line in order to write two letters under it is liable.
Terumas ha'Deshen (63): It seems that one may make the forms of letters in the air. However, a Gadol received from his Rebbeyim that it is forbidden, but he did not give the reason. The Gadol explained that he trains his hand to make this form of writing the letter. Rabah and Rav Yosef suggested that one is liable for Makeh b'Patish because he trains his hand. This is difficult. The Gemara rejected this. If someone saw a craftsman on Shabbos and learned, is he liable?! Abaye and Rava gave other answers. Perhaps training his hand is not enough to obligate a Chatas, but it is forbidden mid'Rabanan. Regarding Shevus, we can distinguish doing an action from inaction. Mid'Oraisa, we would not distinguish like this.
Note: I do not understand why we would not distinguish mid'Oraisa. Chatas is only for one who did an action (Sanhedrin 63a)!
Terumas ha'Deshen: I bring a proof to permit from Or Zaru'a, who forbids writing with his finger in liquids on the table, or scratching a line in dry ashes (Magihah - perhaps this should say earth) or liquids. This shows that we forbid l'Chatchilah only dry earth or liquids, in which the impression is seen for a while. In the air, no impression is seen. One need not be careful about this. If we would not say so, why did he need to mention liquids, if even moving the finger [in the forms of letters] is forbidden?! There is no proof from the Mishnah, which exempts one who wrote with liquids, fruit juice, earth... He is exempt, but it is forbidden. Do not infer that it is permitted in the air. Perhaps it is forbidden even in the air. The Mishnah taught a Chidush that even in these materials, one is exempt.
Shulchan Aruch (OC 340:5): One must be careful not to write with his finger in liquids on the table or in ashes.
Mishnah Berurah (18): Even though there is no Chiyuv Chatas, for he does not write with something that lasts, mid'Rabanan it is forbidden with something that does not last.
Mishnah Berurah (19): We forbid even writing with something that does not last on something that does not last, such as vegetable leaves.
Mishnah Berurah (20): Likewise, one may not mark forms of letters in [condensation on] a glass window pane on a cold day, when it is moist. The Tosefta connotes that if one marked letters with something congealed, e.g. blood, or Chelev, he is liable.
Rema: However, one may make the forms of letters in the air.
Taz (3): Terumas ha'Deshen permits in the air, for the impression is not noticeable. If so, it is permitted not only in the air. Rather, one may form letters even on a dry tablet, for the impression is not noticeable at all. Terumas ha'Deshen connotes like this. He forbids in liquids on a table. This implies that on the table itself is permitted.
Magen Avraham (8): We do not say that he trains his hand to write. Likewise, one may see a craft on Shabbos, even though he learns it.
Machatzis ha'Shekel: This is explicit on 103a.
Note: The Terumas ha'Deshen said that this is not a proof. Perhaps one is exempt, but it is forbidden!
Gra (DH Aval): In Sa'if 5, the Mechaber permits marking a Sefer with his fingernail, for it does not last. All the more so one may make forms in the air!
Mishnah Berurah (21): He marks to hint something to his friend. I say that the Heter to see a craft on Shabbos, even though he learns it, is only if he happened to see it. He must be careful not to speak about it at all with the Nochri [from whom he learns]. On Shabbos, one may not intentionally go to the Nochri's house for this, due to "mi'Mtzo Cheftzecha" (pursuing your desires) (307:1, 306:1).
Mishnah Berurah (22): One who scratches forms on his skin is exempt, for this is not the way of writing. Our version of the Tosefta says oppositely, that one is exempt for tearing and liable for marking. The Rishonim addressed this. One who writes over writing is exempt, for it did not help at all. This is if he wrote the same ink or dye over what was there. If he wrote [black] ink over red dye, he is liable for writing and erasing, for the new writing is superior. Red dye over ink is exempt, for this is detrimental.
Kaf ha'Chayim (43): One who draws a human or animal form on a watermelon rind is liable, for they last a while.
Kaf ha'Chayim (47): Some write secrets with raw milk on paper. It is absorbed and invisible. The recipient puts it near a fire, and he can see what was written. He (the recipient) is not Chayav Chatas for this, but mid'Rabanan it is forbidden, for it is like writing, and all the more so if it is Yad Soledes Bo. Even blank paper has moisture. One who puts a wet peg into an oven to dry is liable for cooking! (74b)
Bi'ur Halachah (DH b'Mashkin): The Rashba (115b DH Ha) says that to be Chayav Chatas, the writing need not last forever. It suffices for it to last some time. People often write things that last only temporarily, e.g. Sifrei Zichronos (chronicles). This is Meleches Machsheves. Tosfos (DH Aval) holds like this. Based on this, surely one who writes with a pencil, like merchants normally write their calculations, is liable. This is unlike writing with lead, which is not considered writing mid'Oraisa (Gitin 19a), for we see that pencils make a lasting impression. The Gemara connotes that one is liable even for coal mixed with water. Pencil writing lasts longer! All the more so one is liable according to the Mechaber, who is Machshir (EH 125:2) a Get written with lead. The Pri Megadim wants to distinguish between Gitin and Shabbos, unlike the Rashba, based on the Rambam and Yerushalmi, who exempt on vegetable leaves, but a Get on olive leaves is Kosher. He overlooked the Tosefta that obligates for Shabbos on olive leaves. One is liable for letters on food, for we find that Rabanan are Machshir a Get on food. Only R. Yosi ha'Glili disqualifies, due to a Drashah, for it is unlike a Sefer. The Mordechai (Shabbos 369) explicitly says that erasing applies to food, so automatically writing applies.