b. Powder and Liquid. The first step in preparing a mix of zinc phosphate
cement is to measure the desired amount of liquid and powder onto the surface of a
clean, cool, dry, glass-mixing slab. The amount of each ingredient depends upon the
amount and the consistency desired. Experience gained in usage of desired
consistencies enables the dental specialist to estimate accurately the amount of powder
used according to the number of drops of liquid dispensed. The estimated powder is
placed on one end of the slab. The powder is divided into quarters. Then, the first
quarter (only) is divided in half (into eighths), and the eighth portion (only) is further
divided in half (into sixteenths). When the process is completed, a total of six portions
of powder are readied for mixing (see figure 1-4). An additional small amount of powder
is often placed on the corner of the slab for use if the estimated powder is insufficient for
the desired mix. The liquid is dispensed with the dropper supplied by the manufacturer.
The required number of drops of liquid is dispensed from the dropper bottle in
accordance with the manufacturer's instructions. The drops are dispensed over a wide
area. The close estimation of powder, according to type of mix and number of drops,
and the small increments will enhance the slow blending of powder. This slows the
setting reaction and enables the user to blend the maximum amount of powder to attain
desired consistency. This will help obtain cement with optimum physical properties.
c. Mixing. Mixing is done by the slow blending of the segments one at a time.
This procedure aids in neutralizing the acid and achieving a smooth consistency. A
considerable portion of the slab is used. Mixing is done with a moderate circular motion
of the spatula blade held flat against the slab. The spatula should be rotated
occasionally to blend the material that collects on the top of the blade. A good rule is to
spatulate the first three segments for about 15 seconds. (See figure 1-4.) The next two
segments should be spatulated for about 20 seconds. The final segment should be
spatulated for 30 to 35 seconds. If this is done, the mixing time is not critical and
completion of the mix will take about 1 1/2 minutes. It is important to reach the desired
consistency by using more powder and not to allow a thinner mix to stiffen by
Figure 1-4. The division of powder into standard portions.