Okay, let's tackle this question about adding quartz to gypsum-bonded investments. Hmm, I remember that gypsum investments are used in dentistry for casting crowns and things. The question is about why quartz is added.

First, the core concept here must relate to the properties of gypsum. Gypsum sets by hydration, forming a hard structure. But maybe it's too brittle? Quartz is a mineral, so adding it might improve the material's strength or thermal stability. Wait, when you cast metal in these investments, the heat from the metal could cause expansion. If the investment doesn't handle thermal expansion well, the mold might crack. So quartz, which has low thermal expansion, could help prevent that.

The correct answer is probably about thermal expansion. Let me think. Gypsum investments without quartz might expand too much when heated, leading to inaccuracies or cracks. Adding quartz would counteract that by providing a more dimensionally stable material. So the answer is likely that quartz is added to control expansion.

Now the options. Let's say the options are A. Increase strength, B. Control expansion, C. Improve surface detail, D. Reduce setting time. Then B is correct. The other options: A might be a distractor, but quartz's main role isn't strength. C, surface detail—maybe, but quartz is more about expansion. D, setting time is controlled by other additives like accelerators or retarders.

Clinical pearl: In dental casting, thermal expansion of the investment is crucial to avoid casting defects. Quartz helps maintain dimensional accuracy by minimizing expansion during heating.

**Core Concept**
Gypsum-bonded investments are used in dental casting to create molds for metal restorations. The addition of quartz addresses thermal expansion during the casting process, a critical factor in preventing dimensional inaccuracies or mold failure.

**Why the Correct Answer is Right**
Quartz has a low coefficient of thermal expansion compared to gypsum. When heated (e.g., by molten metal), gypsum alone undergoes significant expansion, which can distort the mold. Quartz particles act as a filler, reducing overall expansion by their physical presence and chemical inertness. This ensures the mold maintains dimensional stability, preventing cracks or inaccuracies in the final casting.

**Why Each Wrong Option is Incorrect**
**Option A:** Adding quartz does not primarily increase mechanical strength. Gypsum’s strength is enhanced by additives like sodium silicate, not quartz.
**Option C:** Surface detail is improved by the smoothness of the investment mix, not quartz.
**Option D:** Setting time is regulated by accelerators (e.g., sodium chloride) or retarders (e.g., tartaric acid), not quartz.

**Clinical Pearl / High-Yield Fact**
Always remember: in dental casting, *thermal expansion control* is critical. Quartz in gypsum investments prevents over-expansion at high temperatures, ensuring the metal casting fits the prepared tooth accurately. This is a classic exam trap—students often confuse quartz’s role with strength or surface detail.

**Correct Answer: B. Control thermal expansion**

✓ Correct Answer: B. counter balance contraction of gypsum during heating