More or less powerful and/or intelligent elementals exist on the
elemental planes than can be summoned. Various forms of free-willed
elemental plane dwellers will be found described elsewhere in this book.
The outstanding true elementals dwelling on these planes will be
mentioned only briefly hereunder.

Conjured elementals fall into three HD strength categories, and this applies to all types:
 
 

Conjured by spell	16 HD
Conjured by summoning device	12 HD
Conjured through a staff	8 HD

Any creature conjuring an elemental may do so but Once per day per
means per elemental type. For example a M-U with two
appropriate staffs could summon at least two elementals, one through
each staff, once per day, each. Both elementals could not be the same
kind. If the magic-user were able to employ several spells to conjure
elementals, each spell would have to summon a different kind of
elemental. However, if the M-U also had a summoning device and
a staff he could possibly evoke three elementals of the same type.

As elementals are stupid and resent being summoned, the conjuring party
must concentrate upon controlling the creature. Failure to do so will result
in the elemental turning upon the summoner 75% of the time and
attacking. The turning elemental will come directly towards the conjuring
party, attacking anything in its path along the way. Control can never be
regained, and an uncontrolled elemental will always return to its own
plane in three turns after control is lost. If an elemental does not turn (25%
chance), it simply goes immediately to its own plane. Control
concentration requires that the summoning party remain stationary and be
neither physically nor mentally attacked, including attack by missile or
distraction. In any event, only one elemental at a time can be controlled.

Elementals are impervious to attacks by normal weapons and even
magical weapons under +2 bonus. Creatures without magical ability of
some sort cannot harm elementals unless the creatures have four or more
hit dice. Magical ability includes paralysis, poison, acid, breath weapons,
and even the characteristic of not being subject to attack by normal
weapons. Kobolds, goblins, orcs, etc. are all powerless to affect
elementals because they have neither magical property nor four or more
hit dice. Ogres, however, could attack an elemental with effect as they
have the necessary strength (four hit dice in this case). Note, however,
that ifa kobold with a +2 magic sword attacked an elemental the weapon
would be effective.

A conjured elemental can be taken over and controlled by a magic-user
casting a dispel magic spell (ratio dispeller's level over conjuring porty's
level to determine chance of success), and deliberately aiming it at
dispelling the control rather than the elemental. However, if the spell foils,
the effect is to strengthen the elemental to a full 8 points per hit die,
double the controller's ability to concentrate, and make the elemental
resent the one attempting the take-over, so that if it becomes uncontrolled
it will go after that magic-user.

DMG: A holy/unholy word will send any elemental back to its plane.
 

TSR5602 Elements (Air Elemental, Earth Elemental)

<in UA, 21-24 HD elementals are mentioned. full details for these should be provided>
<MM.99: note the reference to "with magical power">
 

At that time elementals were rubbery dime store critters about 65 mm scale, each vaguely reminiscent of the element it represented, the fire figure a red and orange, the water one shades of blue, Can't recall the color of the air one, but it was whirrled I think. The earth elemental figurine was sort of man-like and lumpy perhaps. Memory begins to fail me after some 35 years.

Cheers,
Gary